[GeckoView] Introduce a development mode constant to tell the viewers apart

Currently we have a couple of pre-processor checks, specifically for the GV-viewer, spread throughout the code. This works fine when *building* the viewer, however they're obviously ignored in development mode (i.e. `gulp server`).
This leads to a situation where the GV development viewer, i.e. http://localhost:8888/web/viewer-geckoview.html, behaves subtly different from its built version. This could easily lead to bugs, hence this patch introduces a development mode constant to hopefully improve things here.

Finally, in a follow-up to PR 15842, also ignores the `pageMode`-state since there's no sidebar available.

web/app.js

@@ -466,7 +466,9 @@ const PDFViewerApplication = {
       linkService: pdfLinkService,
       eventBus,
       updateMatchesCountOnProgress:
-        typeof PDFJSDev === "undefined" || !PDFJSDev.test("GECKOVIEW"),
+        typeof PDFJSDev === "undefined"
+          ? !window.isGECKOVIEW
+          : !PDFJSDev.test("GECKOVIEW"),
     });
     this.findController = findController;
 
@@ -1247,14 +1249,18 @@ const PDFViewerApplication = {
               spreadMode = stored.spreadMode | 0;
             }
           }
+          // NOTE: Ignore the pageMode/pageLayout in GeckoView since there's no
+          // sidebar available, nor any UI for changing the Scroll/Spread modes.
+          if (
+            typeof PDFJSDev === "undefined"
+              ? !window.isGECKOVIEW
+              : !PDFJSDev.test("GECKOVIEW")
+          ) {
             // Always let the user preference/view history take precedence.
             if (pageMode && sidebarView === SidebarView.UNKNOWN) {
               sidebarView = apiPageModeToSidebarView(pageMode);
             }
-          // NOTE: Always ignore the pageLayout in GeckoView since there's
-          // no UI available to change Scroll/Spread modes for the user.
             if (
-            (typeof PDFJSDev === "undefined" || !PDFJSDev.test("GECKOVIEW")) &&
               pageLayout &&
               scrollMode === ScrollMode.UNKNOWN &&
               spreadMode === SpreadMode.UNKNOWN
@@ -1265,6 +1271,7 @@ const PDFViewerApplication = {
               // scrollMode = modes.scrollMode;
               spreadMode = modes.spreadMode;
             }
+          }
 
           this.setInitialView(hash, {
             rotation,

web/pdf_scripting_manager.js

@@ -269,7 +269,9 @@ class PDFScriptingManager {
           // NOTE: Always ignore the pageLayout in GeckoView since there's
           // no UI available to change Scroll/Spread modes for the user.
           if (
-            (typeof PDFJSDev !== "undefined" && PDFJSDev.test("GECKOVIEW")) ||
+            (typeof PDFJSDev === "undefined"
+              ? window.isGECKOVIEW
+              : PDFJSDev.test("GECKOVIEW")) ||
             isInPresentationMode
           ) {
             return;

web/viewer-geckoview.js

@@ -68,6 +68,7 @@ function webViewerLoad() {
 
       document.head.append(link);
     }
+    window.isGECKOVIEW = true;
 
     import("pdfjs-web/genericcom.js").then(function (genericCom) {
       PDFViewerApplication.run(config);