Use srcset's resource pixel density to determine intrinsic size

Source/core/rendering/RenderImage.h

Index: Source/core/rendering/RenderImage.h
diff --git a/Source/core/rendering/RenderImage.h b/Source/core/rendering/RenderImage.h
index 1204ea5ed79bd0ae5b6fd0ff4acbe8d51cfbfb96..bdd55666926e613e70c6e9213c79ce5e75683681 100644
--- a/Source/core/rendering/RenderImage.h
+++ b/Source/core/rendering/RenderImage.h
@@ -61,6 +61,9 @@ public:
 
     String altText() const { return m_altText; }
 
+    inline void setImageDevicePixelRatio(float factor) { m_imageDevicePixelRatio = factor; }
+    float imageDevicePixelRatio() const { return m_imageDevicePixelRatio; }
+
 protected:
     virtual bool needsPreferredWidthsRecalculation() const OVERRIDE FINAL;
     virtual RenderBox* embeddedContentBox() const OVERRIDE FINAL;
@@ -112,6 +115,7 @@ private:
     bool m_needsToSetSizeForAltText;
     bool m_didIncrementVisuallyNonEmptyPixelCount;
     bool m_isGeneratedContent;
+    float m_imageDevicePixelRatio;
 
     friend class RenderImageScaleObserver;
 };