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

Source/core/loader/FrameFetchContext.cpp

Index: Source/core/loader/FrameFetchContext.cpp
diff --git a/Source/core/loader/FrameFetchContext.cpp b/Source/core/loader/FrameFetchContext.cpp
index 6b686f761440bbb55ef9c4cda26c31079798bd61..32f354259eae2987bc189fe668027698abfb595c 100644
--- a/Source/core/loader/FrameFetchContext.cpp
+++ b/Source/core/loader/FrameFetchContext.cpp
@@ -616,10 +616,10 @@ 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)));