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

Source/core/loader/FrameFetchContext.cpp

 /*
  * 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 638 matching lines @@
 void FrameFetchContext::countClientHintsResourceWidth()
 {
     UseCounter::count(frame(), UseCounter::ClientHintsResourceWidth);
 }
 
 void FrameFetchContext::countClientHintsViewportWidth()
 {
     UseCounter::count(frame(), UseCounter::ClientHintsViewportWidth);
 }
 
+bool FrameFetchContext::isLowPriorityIframe() const
+{
+    return !frame()->isMainFrame() && frame()->settings() && frame()->settings()->lowPriorityIframes();
+}
+
 DEFINE_TRACE(FrameFetchContext)
 {
     visitor->trace(m_document);
     FetchContext::trace(visitor);
 }
 
 } // namespace blink