Add a Setting to make iframes load at VeryLow priority, disabled by default.

Source/core/loader/FrameFetchContext.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 86 matching lines @@
     bool defersLoading() const override;
     bool isLoadComplete() const override;
     bool pageDismissalEventBeingDispatched() const override;
     bool updateTimingInfoForIFrameNavigation(ResourceTimingInfo*) override;
     void sendImagePing(const KURL&) override;
     void addConsoleMessage(const String&) const override;
     SecurityOrigin* securityOrigin() const override;
     void upgradeInsecureRequest(FetchRequest&) override;
     void addClientHintsIfNecessary(FetchRequest&) override;
     void addCSPHeaderIfNecessary(Resource::Type, FetchRequest&) override;
+    bool isLowPriorityIframe() const override;
 
     virtual void countClientHintsDPR() override;
     virtual void countClientHintsResourceWidth() override;
     virtual void countClientHintsViewportWidth() override;
 
     DECLARE_VIRTUAL_TRACE();
 
 private:
     explicit FrameFetchContext(DocumentLoader*);
     inline DocumentLoader* ensureLoaderForNotifications();
 
     LocalFrame* frame() const; // Can be null
     void printAccessDeniedMessage(const KURL&) const;
 
     // FIXME: Oilpan: Ideally this should just be a traced Member but that will
     // currently leak because ComputedStyle and its data are not on the heap.
     // See crbug.com/383860 for details.
     RawPtrWillBeWeakMember<Document> m_document;
     DocumentLoader* m_documentLoader;
 };
 
 }
 
 #endif