Bug 1742438 - Part 8: Use mScriptData instead of custom load context field; r=arai,asuth

This is the most substantial change in the transition from ScriptLoadInfo to ScriptLoadRequest with
regards to data representation. ScriptLoadRequests can have their data incrementally loaded,
so it is already fully decoded and ready to go by the time that we create the source buffer for
worker scripts. This simplifies some of the code, and we can add incremental loading when we are ready.

Differential Revision: https://phabricator.services.mozilla.com/D146180

js/loader/ScriptLoadRequest.h

@@ -229,11 +229,13 @@ class ScriptLoadRequest
     mDataType = DataType::eUnknown;
     mScriptData.reset();
   }
+
+  bool IsUTF8ParsingEnabled();
+
   void SetTextSource() {
     MOZ_ASSERT(IsUnknownDataType());
     mDataType = DataType::eTextSource;
-    if (mozilla::StaticPrefs::
-            dom_script_loader_external_scripts_utf8_parsing_enabled()) {
+    if (IsUTF8ParsingEnabled()) {
       mScriptData.emplace(VariantType<ScriptTextBuffer<Utf8Unit>>());
     } else {
       mScriptData.emplace(VariantType<ScriptTextBuffer<char16_t>>());