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

Source/core/html/canvas/CanvasRenderingContext2D.h

 /*
  * Copyright (C) 2006, 2007, 2009, 2010, 2011, 2012 Apple 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 240 matching lines @@
         RefPtrWillBeMember<CanvasStyle> m_strokeStyle;
         RefPtrWillBeMember<CanvasStyle> m_fillStyle;
         float m_lineWidth;
         LineCap m_lineCap;
         LineJoin m_lineJoin;
         float m_miterLimit;
         FloatSize m_shadowOffset;
         float m_shadowBlur;
         RGBA32 m_shadowColor;
         float m_globalAlpha;
-        CompositeOperator m_globalComposite;
-        WebBlendMode m_globalBlend;
+        SkXfermode::Mode m_globalComposite;
         AffineTransform m_transform;
         bool m_invertibleCTM;
         Vector<float> m_lineDash;
         float m_lineDashOffset;
         bool m_imageSmoothingEnabled;
 
         // Text state.
         TextAlign m_textAlign;
         TextBaseline m_textBaseline;
         Direction m_direction;
@@ skipping 87 matching lines @@
     Timer<CanvasRenderingContext2D> m_dispatchContextLostEventTimer;
     Timer<CanvasRenderingContext2D> m_dispatchContextRestoredEventTimer;
     Timer<CanvasRenderingContext2D> m_tryRestoreContextEventTimer;
 };
 
 DEFINE_TYPE_CASTS(CanvasRenderingContext2D, CanvasRenderingContext, context, context->is2d(), context.is2d());
 
 } // namespace blink
 
 #endif // CanvasRenderingContext2D_h