Use SkImage as a backing store for copying 2d Contexts to ImageBitmaps.

Source/core/page/ImageBitmap.h

Index: Source/core/page/ImageBitmap.h
diff --git a/Source/core/page/ImageBitmap.h b/Source/core/page/ImageBitmap.h
index 5b91a5f618ae9eb6b5f4909720696cc2e243e19e..585728334b8a3dca16bfbd48e32350538258add3 100644
--- a/Source/core/page/ImageBitmap.h
+++ b/Source/core/page/ImageBitmap.h
@@ -33,6 +33,7 @@ public:
 
     IntRect bitmapRect() const { return m_bitmapRect; }
     IntPoint bitmapOffset() const { return m_bitmapOffset; }
+    bool derivedFromCanvas() const { return m_derivedFromCanvas; }
 
     int width() const { return m_cropRect.width(); }
     int height() const { return m_cropRect.height(); }
@@ -61,10 +62,11 @@ private:
     IntRect m_cropRect;
 
     // The offset by which the desired Image is stored internally.
-    // ImageBitmaps constructed from HTMLImageElements reference the entire CachedImage and may have a non-zero bitmap offset.
-    // ImageBitmaps not constructed from HTMLImageElements always pre-crop and store the image at (0, 0).
+    // ImageBitmaps constructed from image elements or canvases reference the entire CachedImage and may have a non-zero bitmap offset.
+    // Otherwise, ImageBitmaps pre-crop and store the image at (0, 0).
     IntPoint m_bitmapOffset;
 
+    unsigned m_derivedFromCanvas : 1;
 };
 
 } // namespace WebCore