Merge https://codereview.chromium.org/14896002/ to M27: Always use a separate front texture on Mac …

WebCore/platform/graphics/chromium/DrawingBufferChromium.cpp

 /*
  * 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 79 matching lines @@
     , m_multisampleFBO(0)
     , m_multisampleColorBuffer(0)
     , m_contentsChanged(true)
 {
     // Used by browser tests to detect the use of a DrawingBuffer.
     TRACE_EVENT_INSTANT0("test_gpu", "DrawingBufferCreation");
 
     // We need a separate front and back textures if ...
     m_separateFrontTexture = m_preserveDrawingBuffer == Preserve // ... we have to preserve contents after compositing, which is done with a copy or ...
                              || WebKit::Platform::current()->isThreadedCompositingEnabled(); // ... if we're in threaded mode and need to double buffer.
+#if OS(DARWIN)
+    // http://crbug.com/234428 : always use a separate front texture
+    // on Mac OS. Doing this on all GPUs, not just NVIDIA GPUs,
+    // ensures consistent behavior and is much less code.
+    m_separateFrontTexture = true;
+#endif
+
     initialize(size);
 }
 
 DrawingBuffer::~DrawingBuffer()
 {
     if (!m_context)
         return;
 
     clear();
 }
@@ skipping 144 matching lines @@
 void DrawingBuffer::clearPlatformLayer()
 {
     if (m_private)
         m_private->clearTextureId();
 
     m_context->flush();
 }
 #endif
 
 }