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

Source/core/fetch/ResourceFetcher.h

 /*
     Copyright (C) 1998 Lars Knoll (knoll@mpi-hd.mpg.de)
     Copyright (C) 2001 Dirk Mueller <mueller@kde.org>
     Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
     Copyright (C) 2009 Torch Mobile Inc. http://www.torchmobile.com/
 
     This library is free software; you can redistribute it and/or
     modify it under the terms of the GNU Library General Public
     License as published by the Free Software Foundation; either
     version 2 of the License, or (at your option) any later version.
@@ skipping 113 matching lines @@
     enum AccessControlLoggingDecision {
         ShouldLogAccessControlErrors,
         ShouldNotLogAccessControlErrors
     };
     bool canAccessRedirect(Resource*, ResourceRequest&, const ResourceResponse&, ResourceLoaderOptions&);
     bool canAccessResource(Resource*, SecurityOrigin*, const KURL&, AccessControlLoggingDecision) const;
     bool isControlledByServiceWorker() const;
 
     void acceptDataFromThreadedReceiver(unsigned long identifier, const char* data, int dataLength, int encodedDataLength);
 
-    static ResourceLoadPriority loadPriority(Resource::Type, const FetchRequest&);
+    ResourceLoadPriority loadPriority(Resource::Type, const FetchRequest&);
 
     enum ResourceLoadStartType {
         ResourceLoadingFromNetwork,
         ResourceLoadingFromCache
     };
     void requestLoadStarted(Resource*, const FetchRequest&, ResourceLoadStartType);
     static const ResourceLoaderOptions& defaultResourceOptions();
 
     String getCacheIdentifier() const;
 
@@ skipping 87 matching lines @@
             m_loader->m_allowStaleResources = m_previousState;
     }
 private:
     RawPtrWillBeMember<ResourceFetcher> m_loader;
     bool m_previousState;
 };
 
 } // namespace blink
 
 #endif // ResourceFetcher_h