Support the DPR header for client hints.

Source/core/fetch/ImageResource.h

 /*
     Copyright (C) 1998 Lars Knoll (knoll@mpi-hd.mpg.de)
     Copyright (C) 2001 Dirk Mueller <mueller@kde.org>
     Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
     Copyright (C) 2004, 2005, 2006, 2007 Apple Inc. All rights reserved.
 
     This library is free software; you can redistribute it and/or
     modify it under the terms of the GNU Library General Public
     License as published by the Free Software Foundation; either
     version 2 of the License, or (at your option) any later version.
@@ skipping 46 matching lines @@
 
     static std::pair<WebCore::Image*, float> brokenImage(float deviceScaleFactor); // Returns an image and the image's resolution scale factor.
     bool willPaintBrokenImage() const;
 
     bool canRender(const RenderObject* renderer, float multiplier) { return !errorOccurred() && !imageSizeForRenderer(renderer, multiplier).isEmpty(); }
 
     void setContainerSizeForRenderer(const ImageResourceClient*, const IntSize&, float);
     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
         IntrinsicSize // Report the intrinsic size, i.e. ignore whatever has been set extrinsically.
     };
     // This method takes a zoom multiplier that can be used to increase the natural size of the image by the zoom.
     LayoutSize imageSizeForRenderer(const RenderObject*, float multiplier, SizeType = NormalSize); // returns the size of the complete image.
     void computeIntrinsicDimensions(Length& intrinsicWidth, Length& intrinsicHeight, FloatSize& intrinsicRatio);
 
     virtual void didAddClient(ResourceClient*);
@@ skipping 29 matching lines @@
     void updateImage(bool allDataReceived);
     void clearImage();
     // If not null, changeRect is the changed part of the image.
     void notifyObservers(const IntRect* changeRect = 0);
 
     virtual void switchClientsToRevalidatedResource() OVERRIDE;
 
     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;
 };
 
 }
 
 #endif