Support the DPR header for client hints.

Source/core/fetch/ImageResource.h

Index: Source/core/fetch/ImageResource.h
diff --git a/Source/core/fetch/ImageResource.h b/Source/core/fetch/ImageResource.h
index 968a76060572c55c34c64b12f88de081210c0315..00bbb7167f4d846d60b4c1c34233269ccf6710ab 100644
--- a/Source/core/fetch/ImageResource.h
+++ b/Source/core/fetch/ImageResource.h
@@ -64,6 +64,9 @@ public:
     bool usesImageContainerSize() const;
     bool imageHasRelativeWidth() const;
     bool imageHasRelativeHeight() const;
+    // The device pixel resolution we got from the server for this image, or 1.0

[pdr., 2013/10/26 01:26:31]

Nit: change this to:
// The device pixel ratio we got from the server for this image, or 1.0.

+    float dprHeaderValue() const { return m_dprHeaderValue; }

[pdr., 2013/10/26 01:26:31]

Bikeshed: Do you mind spelling dpr out? For example,
"devicePixelRatioHeaderValue()", "m_devicePixelRatioHeaderValue", etc.

I think spelling it out will make this a little easier to understand for
dpr-noobs, and it's consistent with how we named things for srcset.

+    bool hasDprHeaderValue() const { return m_hasDprHeaderValue; }
 
     enum SizeType {
         NormalSize, // Report the size of the image associated with a certain renderer
@@ -113,10 +116,12 @@ private:
     typedef pair<IntSize, float> SizeAndZoom;
     typedef HashMap<const ImageResourceClient*, SizeAndZoom> ContainerSizeRequests;
     ContainerSizeRequests m_pendingContainerSizeRequests;
+    float m_dprHeaderValue;
 
     RefPtr<WebCore::Image> m_image;
     OwnPtr<SVGImageCache> m_svgImageCache;
     bool m_loadingMultipartContent;
+    bool m_hasDprHeaderValue;
 };
 
 }