Added preloader support for sizes based RW values

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 69775e99db5a706201e2c1385c9cafd9967bedcb..7f83f97bf57f12b5cfcc857c3d10fe667adf733e 100644
--- a/Source/core/loader/FrameFetchContext.cpp
+++ b/Source/core/loader/FrameFetchContext.cpp
@@ -666,9 +666,12 @@ void FrameFetchContext::addClientHintsIfNecessary(FetchRequest& fetchRequest)
     if (frame()->shouldSendDPRHint())
         fetchRequest.mutableResourceRequest().addHTTPHeaderField("DPR", AtomicString(String::number(m_document->devicePixelRatio())));
 
-    // FIXME: Send the RW hint based on the actual resource width, when we have it.
-    if (frame()->shouldSendRWHint() && frame()->view())
-        fetchRequest.mutableResourceRequest().addHTTPHeaderField("RW", AtomicString(String::number(frame()->view()->viewportWidth())));
+    if (frame()->shouldSendRWHint() && frame()->view()) {
+        int resourceWidth = fetchRequest.resourceWidth();
+        if (resourceWidth < 0)

[Mike West, 2015/04/09 11:42:48]

Can resourceWidth reasonably be 0? It would be nice if we could make this
unsigned.

+            resourceWidth = frame()->view()->viewportWidth();
+        fetchRequest.mutableResourceRequest().addHTTPHeaderField("RW", AtomicString(String::number(resourceWidth)));
+    }
 }
 
 void FrameFetchContext::addCSPHeaderIfNecessary(Resource::Type type, FetchRequest& fetchRequest)