Replace the BaseFontLoader classes with one FontLoader class

By having just *one* class, and using pre-processor blocks directly in the relevant methods, we reduce the size of this code in the *built* `pdf.js` file.
Originally, when the `BaseFontLoader` abstraction was added in PR 9982, the idea was probably that additional build-targets would get their own implementations. Given that this hasn't happened in the four years since that landed, it doesn't seem meaningful to keep it around.

src/display/font_loader.js

@@ -19,22 +19,17 @@ import {
   FeatureTest,
   shadow,
   string32,
-  unreachable,
   UNSUPPORTED_FEATURES,
   warn,
 } from "../shared/util.js";
 
-class BaseFontLoader {
+class FontLoader {
   constructor({
     docId,
     onUnsupportedFeature,
     ownerDocument = globalThis.document,
-    // For testing only.
+    styleElement = null, // For testing only.
-    styleElement = null,
   }) {
-    if (this.constructor === BaseFontLoader) {
-      unreachable("Cannot initialize BaseFontLoader.");
-    }
     this.docId = docId;
     this._onUnsupportedFeature = onUnsupportedFeature;
     this._document = ownerDocument;
@@ -44,6 +39,11 @@ class BaseFontLoader {
       typeof PDFJSDev === "undefined" || PDFJSDev.test("!PRODUCTION || TESTING")
         ? styleElement
         : null;
+
+    if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("MOZCENTRAL")) {
+      this.loadingRequests = [];
+      this.loadTestFontId = 0;
+    }
   }
 
   addNativeFontFace(nativeFontFace) {
@@ -120,10 +120,6 @@ class BaseFontLoader {
     }
   }
 
-  _queueLoadingCallback(callback) {
-    unreachable("Abstract method `_queueLoadingCallback`.");
-  }
-
   get isFontLoadingAPISupported() {
     const hasFonts = !!this._document?.fonts;
     if (
@@ -139,39 +135,11 @@ class BaseFontLoader {
     return shadow(this, "isFontLoadingAPISupported", hasFonts);
   }
 
-  // eslint-disable-next-line getter-return
   get isSyncFontLoadingSupported() {
-    unreachable("Abstract method `isSyncFontLoadingSupported`.");
-  }
-
-  // eslint-disable-next-line getter-return
-  get _loadTestFont() {
-    unreachable("Abstract method `_loadTestFont`.");
-  }
-
-  _prepareFontLoadEvent(rules, fontsToLoad, request) {
-    unreachable("Abstract method `_prepareFontLoadEvent`.");
-  }
-}
-
-let FontLoader;
     if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) {
-  FontLoader = class MozcentralFontLoader extends BaseFontLoader {
-    get isSyncFontLoadingSupported() {
       return shadow(this, "isSyncFontLoadingSupported", true);
     }
-  };
-} else {
-  // PDFJSDev.test('CHROME || GENERIC')
 
-  FontLoader = class GenericFontLoader extends BaseFontLoader {
-    constructor(params) {
-      super(params);
-      this.loadingRequests = [];
-      this.loadTestFontId = 0;
-    }
-
-    get isSyncFontLoadingSupported() {
     let supported = false;
     if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("CHROME")) {
       if (typeof navigator === "undefined") {
@@ -180,9 +148,7 @@ if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) {
       } else {
         // User agent string sniffing is bad, but there is no reliable way to
         // tell if the font is fully loaded and ready to be used with canvas.
-          const m = /Mozilla\/5.0.*?rv:(\d+).*? Gecko/.exec(
+        const m = /Mozilla\/5.0.*?rv:(\d+).*? Gecko/.exec(navigator.userAgent);
-            navigator.userAgent
-          );
         if (m?.[1] >= 14) {
           supported = true;
         }
@@ -193,6 +159,10 @@ if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) {
   }
 
   _queueLoadingCallback(callback) {
+    if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) {
+      throw new Error("Not implemented: _queueLoadingCallback");
+    }
+
     function completeRequest() {
       assert(!request.done, "completeRequest() cannot be called twice.");
       request.done = true;
@@ -215,10 +185,13 @@ if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) {
   }
 
   get _loadTestFont() {
-      const getLoadTestFont = function () {
+    if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) {
+      throw new Error("Not implemented: _loadTestFont");
+    }
+
     // This is a CFF font with 1 glyph for '.' that fills its entire width
     // and height.
-        return atob(
+    const testFont = atob(
       "T1RUTwALAIAAAwAwQ0ZGIDHtZg4AAAOYAAAAgUZGVE1lkzZwAAAEHAAAABxHREVGABQA" +
         "FQAABDgAAAAeT1MvMlYNYwkAAAEgAAAAYGNtYXABDQLUAAACNAAAAUJoZWFk/xVFDQAA" +
         "ALwAAAA2aGhlYQdkA+oAAAD0AAAAJGhtdHgD6AAAAAAEWAAAAAZtYXhwAAJQAAAAARgA" +
@@ -242,11 +215,14 @@ if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) {
         "Dov6fAH6fAT+fPp8+nwHDosMCvm1Cvm1DAz6fBQAAAAAAAABAAAAAMmJbzEAAAAAzgTj" +
         "FQAAAADOBOQpAAEAAAAAAAAADAAUAAQAAAABAAAAAgABAAAAAAAAAAAD6AAAAAAAAA=="
     );
-      };
+    return shadow(this, "_loadTestFont", testFont);
-      return shadow(this, "_loadTestFont", getLoadTestFont());
   }
 
   _prepareFontLoadEvent(rules, fonts, request) {
+    if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) {
+      throw new Error("Not implemented: _prepareFontLoadEvent");
+    }
+
     /** Hack begin */
     // There's currently no event when a font has finished downloading so the
     // following code is a dirty hack to 'guess' when a font is ready.
@@ -316,8 +292,7 @@ if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) {
     }
     if (i < loadTestFontId.length) {
       // align to 4 bytes boundary
-        checksum =
+      checksum = (checksum - XXXX_VALUE + int32(loadTestFontId + "XXX", i)) | 0;
-          (checksum - XXXX_VALUE + int32(loadTestFontId + "XXX", i)) | 0;
     }
     data = spliceString(data, CFF_CHECKSUM_OFFSET, 4, string32(checksum));
 
@@ -351,8 +326,7 @@ if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) {
     });
     /** Hack end */
   }
-  };
+}
-} // End of PDFJSDev.test('CHROME || GENERIC')
 
 class FontFaceObject {
   constructor(