Merge 76864 - 2011-01-27 Adrienne Walker <enne@google.com>...

Source/WebCore/platform/graphics/chromium/LayerRendererChromium.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 248 matching lines @@
     m_context->clear(GraphicsContext3D::COLOR_BUFFER_BIT);
     // Mask out writes to alpha channel: subpixel antialiasing via Skia results in invalid
     // zero alpha values on text glyphs. The root layer is always opaque.
     m_context->colorMask(true, true, true, false);
 
     updateAndDrawRootLayer(tilePaint, scrollbarPaint, visibleRect, contentRect);
 
     // Re-enable color writes to layers, which may be partially transparent.
     m_context->colorMask(true, true, true, true);
 
+    // Recheck that we still have a root layer.  This may become null if
+    // compositing gets turned off during a paint operation.
+    if (!m_rootLayer)
+        return;
+
     // Set the root visible/content rects --- used by subsequent drawLayers calls.
     m_rootVisibleRect = visibleRect;
     m_rootContentRect = contentRect;
 
     // Scissor out the scrollbars to avoid rendering on top of them.
     IntRect rootScissorRect(contentRect);
     // The scissorRect should not include the scroll offset.
     rootScissorRect.move(-m_scrollPosition.x(), -m_scrollPosition.y());
     m_rootLayer->m_scissorRect = rootScissorRect;
 
@@ skipping 517 matching lines @@
     m_rootLayerTiler.clear();
     m_horizontalScrollbarTiler.clear();
     m_verticalScrollbarTiler.clear();
 
     m_textureManager.clear();
 }
 
 } // namespace WebCore
 
 #endif // USE(ACCELERATED_COMPOSITING)