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 67d374af52a8b9630e63aee72d23b573d9acd0b5..d168dc759aaf9c9234f77807cd39d2b94d9593fb 100644
--- a/Source/core/html/HTMLCanvasElement.h
+++ b/Source/core/html/HTMLCanvasElement.h
@@ -114,7 +114,7 @@ public:
     ImageBuffer* buffer() const;
     Image* copiedImage() const;
     void clearCopiedImage();
-    PassRefPtrWillBeRawPtr<ImageData> getImageData();
+    PassRefPtrWillBeRawPtr<ImageData> getImageData() const;
     void makePresentationCopy();
     void clearPresentationCopy();
 
@@ -132,6 +132,8 @@ public:
 
     bool shouldAccelerate(const IntSize&) const;
 
+    virtual const AtomicString imageSourceURL() const OVERRIDE;
+
     virtual InsertionNotificationRequest insertedInto(ContainerNode*) OVERRIDE;
 
     // DocumentVisibilityObserver implementation
@@ -168,6 +170,8 @@ private:
 
     void updateExternallyAllocatedMemory() const;
 
+    String toDataURLInternal(const String& mimeType, const double* quality) const;
+
     HashSet<CanvasObserver*> m_observers;
 
     IntSize m_size;