Add Accept-CH meta http-equiv support to the preloader.

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 598 matching lines @@
             fetchRequest.mutableResourceRequest().setURL(url);
         }
     }
 }
 
 void FrameFetchContext::addClientHintsIfNecessary(FetchRequest& fetchRequest)
 {
     if (!RuntimeEnabledFeatures::clientHintsEnabled() || !m_document)
         return;
 
-    if (m_document->clientHintsPreferences().shouldSendDPR())
+    if (m_document->clientHintsPreferences().shouldSendDPR() || fetchRequest.clientHintsPreferences().shouldSendDPR())

[Mike West, 2015/05/05 15:22:36]

I'd suggest extracting this into a `shouldSendDPR` boolean variable, and reusing
it here and below for clarity.

         fetchRequest.mutableResourceRequest().addHTTPHeaderField("DPR", AtomicString(String::number(m_document->devicePixelRatio())));
 
-    if (m_document->clientHintsPreferences().shouldSendRW() && frame()->view()) {
+    if ((m_document->clientHintsPreferences().shouldSendRW() || fetchRequest.clientHintsPreferences().shouldSendRW()) && frame()->view()) {
         FetchRequest::ResourceWidth resourceWidth = fetchRequest.resourceWidth();
         float usedResourceWidth = resourceWidth.isSet ? resourceWidth.width : frame()->view()->viewportWidth();
         fetchRequest.mutableResourceRequest().addHTTPHeaderField("RW", AtomicString(String::number(usedResourceWidth)));
     }
 }
 
 void FrameFetchContext::addCSPHeaderIfNecessary(Resource::Type type, FetchRequest& fetchRequest)
 {
     if (!m_document)
         return;
 
     const ContentSecurityPolicy* csp = m_document->contentSecurityPolicy();
     if (csp->shouldSendCSPHeader(type))
         fetchRequest.mutableResourceRequest().addHTTPHeaderField("CSP", "active");
 }
 
 DEFINE_TRACE(FrameFetchContext)
 {
     visitor->trace(m_document);
     FetchContext::trace(visitor);
 }
 
 } // namespace blink