Add canvas blending modes under a runtime flag

Source/core/platform/graphics/skia/ImageBufferSkia.cpp

 /*
  * Copyright (c) 2008, Google Inc. All rights reserved.
  * Copyright (C) 2009 Dirk Schulze <krit@webkit.org>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. 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
@@ skipping 240 matching lines @@
 {
     context->beginLayerClippedToImage(rect, this);
 }
 
 static bool drawNeedsCopy(GraphicsContext* src, GraphicsContext* dst)
 {
     return (src == dst);
 }
 
 void ImageBuffer::draw(GraphicsContext* context, ColorSpace styleColorSpace, const FloatRect& destRect, const FloatRect& srcRect,
-    CompositeOperator op, BlendMode, bool useLowQualityScale)
+    CompositeOperator op, BlendMode blendMode, bool useLowQualityScale)
 {
     const SkBitmap& bitmap = *m_context->bitmap();
     RefPtr<Image> image = BitmapImage::create(NativeImageSkia::create(drawNeedsCopy(m_context.get(), context) ? deepSkBitmapCopy(bitmap) : bitmap));
-    context->drawImage(image.get(), styleColorSpace, destRect, srcRect, op, DoNotRespectImageOrientation, useLowQualityScale);
+    context->drawImage(image.get(), styleColorSpace, destRect, srcRect, op, blendMode, DoNotRespectImageOrientation, useLowQualityScale);
 }
 
 void ImageBuffer::drawPattern(GraphicsContext* context, const FloatRect& srcRect, const AffineTransform& patternTransform,
                               const FloatPoint& phase, ColorSpace styleColorSpace, CompositeOperator op, const FloatRect& destRect)
 {
     const SkBitmap& bitmap = *m_context->bitmap();
     RefPtr<Image> image = BitmapImage::create(NativeImageSkia::create(drawNeedsCopy(m_context.get(), context) ? deepSkBitmapCopy(bitmap) : bitmap));
     image->drawPattern(context, srcRect, patternTransform, phase, styleColorSpace, op, destRect);
 }
 
@@ skipping 160 matching lines @@
     if (!encodeImage(imageData, mimeType, quality, &encodedImage))
         return "data:,";
 
     Vector<char> base64Data;
     base64Encode(encodedImage, base64Data);
 
     return "data:" + mimeType + ";base64," + base64Data;
 }
 
 } // namespace WebCore