Use glDiscardFramebuffer to save memory bandwidth

Source/platform/graphics/gpu/DrawingBufferTest.cpp

 /*
  * 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:
  *
  *     * 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 172 matching lines @@
             return PassRefPtr<DrawingBufferForTests>();
         }
         return drawingBuffer.release();
     }
 
     DrawingBufferForTests(PassOwnPtr<WebGraphicsContext3D> context,
         PassOwnPtr<Extensions3DUtil> extensionsUtil,
         PreserveDrawingBuffer preserve,
         PassRefPtr<ContextEvictionManager> contextEvictionManager)
         : DrawingBuffer(context, extensionsUtil, false /* multisampleExtensionSupported */,
-            false /* packedDepthStencilExtensionSupported */, preserve, WebGraphicsContext3D::Attributes(), contextEvictionManager)
+            false /* packedDepthStencilExtensionSupported */, false /* discardFramebufferSupported */, preserve, WebGraphicsContext3D::Attributes(), contextEvictionManager)
         , m_live(0)
     { }
 
     virtual ~DrawingBufferForTests()
     {
         if (m_live)
             *m_live = false;
     }
 
     bool* m_live;
@@ skipping 466 matching lines @@
     mailbox.syncPoint = waitSyncPoint;
     m_drawingBuffer->setIsHidden(true);
     m_drawingBuffer->mailboxReleased(mailbox);
     // m_drawingBuffer deletes mailbox immediately when hidden.
     EXPECT_EQ(waitSyncPoint, webContext()->mostRecentlyWaitedSyncPoint());
 
     m_drawingBuffer->beginDestruction();
 }
 
 } // namespace