[Firefox viewer] Skip some unnecessary code in the FontLoader.bind method

Given that Firefox supports *synchronous* font loading, when the Font Loading API isn't being used, there's really no point including code which if called would just throw in the MOZCENTRAL build. (This is safe, since the `FontLoader.isSyncFontLoadingSupported`-getter always return `true` there.)

src/display/font_loader.js

@@ -112,6 +112,9 @@ class FontLoader {
       if (this.isSyncFontLoadingSupported) {
         return; // The font was, synchronously, loaded.
       }
+      if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) {
+        throw new Error("Not implemented: async font loading");
+      }
       await new Promise(resolve => {
         const request = this._queueLoadingCallback(resolve);
         this._prepareFontLoadEvent(font, request);