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

WebCore/platform/graphics/chromium/DrawingBufferChromium.cpp

Index: WebCore/platform/graphics/chromium/DrawingBufferChromium.cpp
===================================================================
--- WebCore/platform/graphics/chromium/DrawingBufferChromium.cpp	(revision 149621)
+++ WebCore/platform/graphics/chromium/DrawingBufferChromium.cpp	(working copy)
@@ -97,6 +97,13 @@
     // 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);
 }