Add canvas blending modes under a runtime flag

Source/WebCore/platform/graphics/skia/GraphicsContextSkia.cpp

 /*
  * Copyright (c) 2006, 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 875 matching lines @@
 }
 
 void GraphicsContext::setAlpha(float alpha)
 {
     if (paintingDisabled())
         return;
 
     platformContext()->setAlpha(alpha);
 }
 
-void GraphicsContext::setPlatformCompositeOperation(CompositeOperator op, BlendMode)
+void GraphicsContext::setPlatformCompositeOperation(CompositeOperator op, BlendMode blendMode)
 {
     if (paintingDisabled())
         return;
-
-    platformContext()->setXfermodeMode(WebCoreCompositeToSkiaComposite(op));
+    platformContext()->setXfermodeMode(WebCoreCompositeToSkiaComposite(op, blendMode));
 }
 
 InterpolationQuality GraphicsContext::imageInterpolationQuality() const
 {
     return platformContext()->interpolationQuality();
 }
 
 void GraphicsContext::setImageInterpolationQuality(InterpolationQuality q)
 {
     platformContext()->setInterpolationQuality(q);
@@ skipping 312 matching lines @@
     if (paintingDisabled())
         return;
 
     SkRect rect(ellipse);
     SkPaint paint;
     platformContext()->setupPaintForStroking(&paint, 0, 0);
     platformContext()->drawOval(rect, paint);
 }
 
 }  // namespace WebCore