Add canvas blending modes under a runtime flag

Source/core/platform/graphics/GraphicsContext.cpp

 /*
  * Copyright (C) 2003, 2004, 2005, 2006, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 1803 matching lines @@
 }
 
 void GraphicsContext::setCompositeOperation(CompositeOperator compositeOperation, BlendMode blendMode)
 {
     m_state.compositeOperator = compositeOperation;
     m_state.blendMode = blendMode;
 
     if (paintingDisabled())
         return;
 
-    m_skiaState->m_xferMode = WebCoreCompositeToSkiaComposite(compositeOperation);
+    m_skiaState->m_xferMode = WebCoreCompositeToSkiaComposite(compositeOperation, blendMode);
 }
 
 CompositeOperator GraphicsContext::compositeOperation() const
 {
     return m_state.compositeOperator;
 }
 
 BlendMode GraphicsContext::blendModeOperation() const
 {
     return m_state.blendMode;
@@ skipping 286 matching lines @@
     SkPaint paint;
     paint.setXfermodeMode(SkXfermode::kDstIn_Mode);
     realizeSave(SkCanvas::kMatrixClip_SaveFlag);
     m_canvas->save(SkCanvas::kMatrix_SaveFlag);
     m_canvas->resetMatrix();
     m_canvas->drawBitmapRect(imageBuffer, 0, rect, &paint);
     m_canvas->restore();
 }
 
 }