Make beginLayer() and CanvasRenderingContext2D use SkXfermode::Mode.

Source/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 31 matching lines @@
 #include "platform/graphics/Color.h"
 #include "platform/graphics/GraphicsTypes.h"
 #include "platform/transforms/AffineTransform.h"
 #include "wtf/MathExtras.h"
 
 namespace blink {
 
 class GraphicsContext;
 
 SkXfermode::Mode PLATFORM_EXPORT WebCoreCompositeToSkiaComposite(CompositeOperator, WebBlendMode = WebBlendModeNormal);
+CompositeOperator PLATFORM_EXPORT compositeOperatorFromSkia(SkXfermode::Mode);
+WebBlendMode PLATFORM_EXPORT blendModeFromSkia(SkXfermode::Mode);
 
 inline SkPaint::FilterLevel WebCoreInterpolationQualityToSkFilterLevel(InterpolationQuality quality)
 {
     // FIXME: this reflects existing client mappings, but should probably
     // be expanded to map higher level interpolations more accurately.
     return quality != InterpolationNone ? SkPaint::kLow_FilterLevel : SkPaint::kNone_FilterLevel;
 }
 
 // Skia has problems when passed infinite, etc floats, filter them to 0.
 inline SkScalar WebCoreFloatToSkScalar(float f)
@@ skipping 37 matching lines @@
     float srcHeight,
     float destWidth,
     float destHeight,
     bool isDataComplete = true);
 
 bool shouldDrawAntiAliased(const GraphicsContext*, const SkRect& destRect);
 
 } // namespace blink
 
 #endif  // SkiaUtils_h