Use glDiscardFramebuffer to save memory bandwidth

Source/core/html/canvas/WebGLRenderingContextBase.cpp

 /*
  * Copyright (C) 2009 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 796 matching lines @@
             canvas()->didDraw(FloatRect(FloatPoint(0, 0), clampedCanvasSize()));
         }
     }
 }
 
 bool WebGLRenderingContextBase::clearIfComposited(GLbitfield mask)
 {
     if (isContextLost())
         return false;
 
-    if (!drawingBuffer()->layerComposited() || m_layerCleared
+    if (!drawingBuffer()->layerComposited() || drawingBuffer()->discardFramebufferSupported() || m_layerCleared
         || m_requestedAttributes->preserveDrawingBuffer() || (mask && m_framebufferBinding))
         return false;
 
     RefPtrWillBeRawPtr<WebGLContextAttributes> contextAttributes = getContextAttributes();
 
     // Determine if it's possible to combine the clear the user asked for and this clear.
     bool combinedClear = mask && !m_scissorEnabled;
 
     webContext()->disable(GL_SCISSOR_TEST);
     if (combinedClear && (mask & GL_COLOR_BUFFER_BIT)) {
@@ skipping 4966 matching lines @@
     return m_sharedWebGraphicsContext3D ? m_sharedWebGraphicsContext3D->drawingBuffer() : 0;
 }
 #else
 DrawingBuffer* WebGLRenderingContextBase::drawingBuffer() const
 {
     return m_drawingBuffer.get();
 }
 #endif
 
 } // namespace blink