Add GL_CHROMIUM_screen_space_antialiasing to support alternative AA

Source/platform/graphics/gpu/DrawingBuffer.h

 /*
  * Copyright (c) 2010, 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 267 matching lines @@
     bool m_contentsChanged;
 
     // True if commit() has been called since the last time markContentsChanged() had been called.
     bool m_contentsChangeCommitted;
     bool m_bufferClearNeeded;
 
     enum MultisampleMode {
         None,
         ImplicitResolve,
         ExplicitResolve,
+        INTELCMAAResolve,

[bajones, 2015/08/19 16:26:44]

Nit: I know that you're following the capitalization pattern from the extension
here, but the Blink style guide indicates this should be "IntelCMAAResolve"

     };
 
     MultisampleMode m_multisampleMode;
 
     WebGraphicsContext3D::Attributes m_actualAttributes;
     unsigned m_internalColorFormat;
     unsigned m_colorFormat;
     unsigned m_internalRenderbufferFormat;
     int m_maxTextureSize;
     int m_sampleCount;
@@ skipping 16 matching lines @@
     // used to resize the Canvas.
     SkBitmap m_resizingBitmap;
 
     // Used to flip a bitmap vertically.
     Vector<uint8_t> m_scanline;
 };
 
 } // namespace blink
 
 #endif // DrawingBuffer_h