Implement CSS luminance masking using skia SkLumaMaskXfermode class.

Source/core/platform/graphics/GraphicsContext.cpp

 /*
  * Copyright (C) 2003, 2004, 2005, 2006, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2013 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:
  * 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
@@ skipping 1695 matching lines @@
     {
         SkPaint defaultPaint;
         SkASSERT(*paint == defaultPaint);
     }
 #endif
 
     paint->setAntiAlias(m_state->m_shouldAntialias);
 
     if (!SkXfermode::IsMode(m_state->m_xferMode.get(), SkXfermode::kSrcOver_Mode))
         paint->setXfermode(m_state->m_xferMode.get());
+    if (this->drawLuminanceMask())
+        paint->setXfermode(SkLumaMaskXfermode::Create(SkXfermode::kSrcOver_Mode));
 
     if (m_state->m_looper)
         paint->setLooper(m_state->m_looper.get());
 
     paint->setColorFilter(m_state->m_colorFilter.get());
 }
 
 void GraphicsContext::drawOuterPath(const SkPath& path, SkPaint& paint, int width)
 {
 #if OS(MACOSX)
@@ skipping 149 matching lines @@
 
 void GraphicsContext::didDrawTextInRect(const SkRect& textRect)
 {
     if (m_trackTextRegion) {
         TRACE_EVENT0("skia", "PlatformContextSkia::trackTextRegion");
         m_textRegion.join(textRect);
     }
 }
 
 }