Fix crash in updateCompositorResources due to null root layer

Source/WebCore/platform/graphics/chromium/LayerRendererChromium.cpp

Index: Source/WebCore/platform/graphics/chromium/LayerRendererChromium.cpp
===================================================================
--- Source/WebCore/platform/graphics/chromium/LayerRendererChromium.cpp	(revision 94434)
+++ Source/WebCore/platform/graphics/chromium/LayerRendererChromium.cpp	(working copy)
@@ -198,7 +198,7 @@
 {
     if (m_rootCCLayerImpl)
         clearRenderSurfacesOnCCLayerImplRecursive(m_rootCCLayerImpl.get());
-    m_computedRenderSurfaceLayerList.clear();
+    m_computedRenderSurfaceLayerList = adoptPtr(new LayerList());
     m_rootCCLayerImpl.clear();
 }
 
@@ -304,8 +304,13 @@
         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()
@@ -381,6 +386,10 @@
         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);
 }