Bug 1814703 - Allow selecting between ARM and x86 clearkey on Windows ARM. r=azebrowski

On Windows ARM, we currently use the x86 version of the clearkey plugin.
We do however ship Firefox with the ARM version although prior to this
patch it was unused. Now if media.gmp-clearkey.allow-x64-plugin-on-arm64
is set to false, then we will attempt to load the ARM version of the
plugin. The pref remains true by default.

Differential Revision: https://phabricator.services.mozilla.com/D168707

toolkit/modules/GMPUtils.sys.mjs

@@ -122,6 +122,7 @@ export var GMPPrefs = {
   KEY_PLUGIN_VISIBLE: "media.{0}.visible",
   KEY_PLUGIN_ABI: "media.{0}.abi",
   KEY_PLUGIN_FORCE_SUPPORTED: "media.{0}.forceSupported",
+  KEY_PLUGIN_ALLOW_X64_ON_ARM64: "media.{0}.allow-x64-plugin-on-arm64",
   KEY_URL: "media.gmp-manager.url",
   KEY_URL_OVERRIDE: "media.gmp-manager.url.override",
   KEY_CERT_CHECKATTRS: "media.gmp-manager.cert.checkAttributes",

toolkit/mozapps/extensions/internal/GMPProvider.sys.mjs

@@ -770,7 +770,14 @@ var GMPProvider = {
     try {
       let greDir = Services.dirsvc.get(NS_GRE_DIR, Ci.nsIFile);
       let path = greDir.path;
-      if (GMPUtils._isWindowsOnARM64()) {
+      if (
+        GMPUtils._isWindowsOnARM64() &&
+        GMPPrefs.getBool(
+          GMPPrefs.KEY_PLUGIN_ALLOW_X64_ON_ARM64,
+          true,
+          CLEARKEY_PLUGIN_ID
+        )
+      ) {
         path = PathUtils.join(path, "i686");
       }
       let clearkeyPath = PathUtils.join(