Fix crash in updateCompositorResources due to null root layer

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 180 matching lines @@
 {
     for (size_t i = 0; i < layer->children().size(); ++i)
         clearRenderSurfacesOnCCLayerImplRecursive(layer->children()[i].get());
     layer->clearRenderSurface();
 }
 
 void LayerRendererChromium::clearRootCCLayerImpl()
 {
     if (m_rootCCLayerImpl)
         clearRenderSurfacesOnCCLayerImplRecursive(m_rootCCLayerImpl.get());
-    m_computedRenderSurfaceLayerList.clear();
+    m_computedRenderSurfaceLayerList = adoptPtr(new LayerList());
     m_rootCCLayerImpl.clear();
 }
 
 GraphicsContext3D* LayerRendererChromium::context()
 {
     return m_context.get();
 }
 
 #if USE(SKIA)
 GrContext* LayerRendererChromium::skiaContext()
@@ skipping 85 matching lines @@
     // 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;
 
     {
         TRACE_EVENT("LayerRendererChromium::synchronizeTrees", this, 0);
         m_rootCCLayerImpl = TreeSynchronizer::synchronizeTrees(m_rootLayer.get(), m_rootCCLayerImpl.get());
     }
 
-    m_computedRenderSurfaceLayerList = adoptPtr(new LayerList());
-    updateLayers(*m_computedRenderSurfaceLayerList);
+    OwnPtr<LayerList> temporaryLayerList = adoptPtr(new LayerList());
+    updateLayers(*temporaryLayerList);
+
+    if (m_rootLayer)
+        m_computedRenderSurfaceLayerList = temporaryLayerList.release();
+    else
+        m_computedRenderSurfaceLayerList = adoptPtr(new LayerList());
 }
 
 void LayerRendererChromium::drawLayers()
 {
     ASSERT(m_hardwareCompositing);
     ASSERT(m_computedRenderSurfaceLayerList);
     // Before drawLayers:
     if (hardwareCompositing()) {
         // FIXME: The multithreaded compositor case will not work as long as
         // copyTexImage2D resolves to the parent texture, because the main
@@ skipping 55 matching lines @@
     paintLayerContents(renderSurfaceLayerList);
 #ifndef NDEBUG
     s_inPaintLayerContents = false;
 #endif
     // Update compositor resources for root layer.
     {
         TRACE_EVENT("LayerRendererChromium::updateLayer::updateRoot", this, 0);
         m_rootLayerContentTiler->updateRect(m_rootLayerTextureUpdater.get());
     }
 
+    // Painting could turn off compositing, so check for the root layer.
+    if (!m_rootLayer)
+        return;
+
     m_contentsTextureManager->reduceMemoryToLimit(textureMemoryReclaimLimitBytes);
     updateCompositorResources(renderSurfaceLayerList);
 }
 
 static IntRect calculateVisibleRect(const IntRect& targetSurfaceRect, const IntRect& layerBoundRect, const TransformationMatrix& transform)
 {
     // Is this layer fully contained within the target surface?
     IntRect layerInSurfaceSpace = transform.mapRect(layerBoundRect);
     if (targetSurfaceRect.contains(layerInSurfaceSpace))
         return layerBoundRect;
@@ skipping 1082 matching lines @@
 PassOwnPtr<CCLayerTreeHostImplProxy> LayerRendererChromium::createLayerTreeHostImplProxy()
 {
     OwnPtr<CCLayerTreeHostImplProxy> proxy = LayerRendererChromiumImplProxy::create(this);
     proxy->start();
     return proxy.release();
 }
 
 } // namespace WebCore
 
 #endif // USE(ACCELERATED_COMPOSITING)