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

Source/core/loader/LinkLoader.cpp

 /*
  * Copyright (C) 2011 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 160 matching lines @@
             document.addConsoleMessage(ConsoleMessage::create(OtherMessageSource, WarningMessageLevel, String("<link rel=preload> has an invalid `href` value")));
             return;
         }
         // TODO(yoav): Figure out a way that 'as' would be used to set request headers.
         Resource::Type priorityType;
         if (!getPriorityTypeFromAsAttribute(as, priorityType)) {
             document.addConsoleMessage(ConsoleMessage::create(OtherMessageSource, WarningMessageLevel, String("<link rel=preload> must have a valid `as` value")));
             return;
         }
         FetchRequest linkRequest(ResourceRequest(document.completeURL(href)), FetchInitiatorTypeNames::link);
-        linkRequest.setPriority(ResourceFetcher::loadPriority(priorityType, linkRequest));
+        linkRequest.setPriority(document.fetcher()->loadPriority(priorityType, linkRequest));
         Settings* settings = document.settings();
         if (settings && settings->logPreload())
             document.addConsoleMessage(ConsoleMessage::create(OtherMessageSource, DebugMessageLevel, String("Preload triggered for " + href.host() + href.path())));
         setResource(LinkFetchResource::fetch(Resource::LinkPreload, linkRequest, document.fetcher()));
     }
 }
 
 bool LinkLoader::loadLinkFromHeader(const String& headerValue, Document* document)
 {
     if (!document)
@@ skipping 63 matching lines @@
         m_prerender.clear();
     }
 }
 
 DEFINE_TRACE(LinkLoader)
 {
     visitor->trace(m_prerender);
 }
 
 }