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

Source/core/fetch/ResourceFetcher.cpp

 /*
     Copyright (C) 1998 Lars Knoll (knoll@mpi-hd.mpg.de)
     Copyright (C) 2001 Dirk Mueller (mueller@kde.org)
     Copyright (C) 2002 Waldo Bastian (bastian@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
@@ skipping 46 matching lines @@
 namespace blink {
 
 ResourceLoadPriority ResourceFetcher::loadPriority(Resource::Type type, const FetchRequest& request)
 {
     // TODO(yoav): Change it here so that priority can be changed even after it was resolved.
     if (request.priority() != ResourceLoadPriorityUnresolved)
         return request.priority();
 
     switch (type) {
     case Resource::MainResource:
-        return ResourceLoadPriorityVeryHigh;
+        return context().isLowPriorityIframe() ? ResourceLoadPriorityVeryLow : ResourceLoadPriorityVeryHigh;
     case Resource::CSSStyleSheet:
         return ResourceLoadPriorityHigh;
     case Resource::Raw:
         return request.options().synchronousPolicy == RequestSynchronously ? ResourceLoadPriorityVeryHigh : ResourceLoadPriorityMedium;
     case Resource::Script:
         // Async scripts do not block the parser so they get the lowest priority and can be
         // loaded in parser order with images.
         if (FetchRequest::LazyLoad == request.defer())
             return ResourceLoadPriorityLow;
         return ResourceLoadPriorityMedium;
@@ skipping 974 matching lines @@
 
 DEFINE_TRACE(ResourceFetcher)
 {
     visitor->trace(m_context);
     visitor->trace(m_archiveResourceCollection);
     visitor->trace(m_loaders);
     visitor->trace(m_nonBlockingLoaders);
 }
 
 }