Early terminate WebGL markContextChanged calls when appropriate.

third_party/WebKit/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 158 matching lines @@
     // Track the currently active texture unit. Texture unit 0 is used as host for a scratch
     // texture.
     void setActiveTextureUnit(GLint textureUnit) { m_activeTextureUnit = textureUnit; }
 
     bool multisample() const;
 
     GLuint framebuffer() const;
 
     bool discardFramebufferSupported() const { return m_discardFramebufferSupported; }
 
-    void markContentsChanged();
+    // Returns false if the contents had previously been marked as changed and
+    // have not yet been comitted.
+    bool markContentsChanged();
     void setBufferClearNeeded(bool);
     bool bufferClearNeeded() const;
     void setIsHidden(bool);
     void setFilterQuality(SkFilterQuality);
 
     // Whether the target for draw operations has format GL_RGBA, but is
     // emulating format GL_RGB. When the target's storage is first
     // allocated, its alpha channel must be cleared to 1. All future drawing
     // operations must use a color mask with alpha=GL_FALSE.
     bool requiresAlphaChannelToBePreserved();
@@ skipping 255 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