Add canvas blending modes under a runtime flag

Source/core/platform/graphics/skia/SkiaUtils.h

 /*
  * Copyright (c) 2006,2007,2008, 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 25 matching lines @@
 #include "SkPath.h"
 #include "SkXfermode.h"
 #include "core/platform/graphics/GraphicsContext.h"
 #include <wtf/MathExtras.h>
 
 class SkCanvas;
 class SkRegion;
 
 namespace WebCore {
 
-SkXfermode::Mode WebCoreCompositeToSkiaComposite(CompositeOperator);
+SkXfermode::Mode WebCoreCompositeToSkiaComposite(CompositeOperator, BlendMode = BlendModeNormal);
 
 // move this guy into SkColor.h
 SkColor SkPMColorToColor(SkPMColor);
 
 // This should be an operator on Color
 Color SkPMColorToWebCoreColor(SkPMColor);
 
 // Skia has problems when passed infinite, etc floats, filter them to 0.
 inline SkScalar WebCoreFloatToSkScalar(float f)
 {
@@ skipping 22 matching lines @@
 bool SkPathContainsPoint(SkPath*, const FloatPoint&, SkPath::FillType);
 
 // Returns a statically allocated 1x1 GraphicsContext intended for temporary
 // operations. Please save() the state and restore() it when you're done with
 // the context.
 GraphicsContext* scratchContext();
 
 }  // namespace WebCore
 
 #endif  // SkiaUtils_h