Implement "Save image as" for canvas (blink side).

Source/core/rendering/RenderHTMLCanvas.h

Index: Source/core/rendering/RenderHTMLCanvas.h
diff --git a/Source/core/rendering/RenderHTMLCanvas.h b/Source/core/rendering/RenderHTMLCanvas.h
index c06ee44cefc2f7dbb5b57087eb209a0ed839412f..816e9c9bcf83678f9a37c1e8f65cc879646dae16 100644
--- a/Source/core/rendering/RenderHTMLCanvas.h
+++ b/Source/core/rendering/RenderHTMLCanvas.h
@@ -42,6 +42,7 @@ public:
     void canvasSizeChanged();
 
 private:
+    virtual bool isImage() const OVERRIDE { return true; }

[Justin Novosad, 2014/04/24 14:19:39]

This is not a good idea. It implies that the object is an HTML image, which it
is not.  Instead you should change the relevant call sites to (isImage() ||
isCanvas())

     virtual const char* renderName() const OVERRIDE { return "RenderHTMLCanvas"; }
     virtual void paintReplaced(PaintInfo&, const LayoutPoint&) OVERRIDE;
     virtual void intrinsicSizeChanged() OVERRIDE { canvasSizeChanged(); }