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

Source/core/html/HTMLCanvasElement.h

Index: Source/core/html/HTMLCanvasElement.h
diff --git a/Source/core/html/HTMLCanvasElement.h b/Source/core/html/HTMLCanvasElement.h
index 634a27e5fa6ddbb43feeb883dc97783f08ce7065..4b0f62558c42fac7a1069192405d8a58be41c2d5 100644
--- a/Source/core/html/HTMLCanvasElement.h
+++ b/Source/core/html/HTMLCanvasElement.h
@@ -97,8 +97,8 @@ public:
     CanvasRenderingContext* getContext(const String&, CanvasContextAttributes* attributes = 0);
 
     static String toEncodingMimeType(const String& mimeType);
-    String toDataURL(const String& mimeType, const double* quality, ExceptionState&);
-    String toDataURL(const String& mimeType, ExceptionState& exceptionState) { return toDataURL(mimeType, 0, exceptionState); }
+    String toDataURL(const String& mimeType, const double* quality, ExceptionState&) const;
+    String toDataURL(const String& mimeType, ExceptionState& exceptionState) const { return toDataURL(mimeType, 0, exceptionState); }
 
     // Used for rendering
     void didDraw(const FloatRect&);
@@ -115,7 +115,7 @@ public:
     ImageBuffer* buffer() const;
     Image* copiedImage() const;
     void clearCopiedImage();
-    PassRefPtrWillBeRawPtr<ImageData> getImageData();
+    PassRefPtrWillBeRawPtr<ImageData> getImageData() const;
     void makePresentationCopy();
     void clearPresentationCopy();
 
@@ -133,6 +133,8 @@ public:
 
     bool shouldAccelerate(const IntSize&) const;
 
+    virtual const AtomicString imageSourceURL() const OVERRIDE;
+
     virtual InsertionNotificationRequest insertedInto(ContainerNode*) OVERRIDE;
 
     // DocumentVisibilityObserver implementation
@@ -171,6 +173,8 @@ private:
 
     void updateExternallyAllocatedMemory() const;
 
+    String toDataURLInternal(const String& mimeType, const double* quality) const;
+
     HashSet<CanvasObserver*> m_observers;
 
     IntSize m_size;