Fix stale compositor references from ui::Layer

ui/gfx/compositor/compositor_cc.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/gfx/compositor/compositor_cc.h"
 
 #include "third_party/skia/include/images/SkImageEncoder.h"
 #include "third_party/skia/include/core/SkBitmap.h"
 #include "third_party/WebKit/Source/WebKit/chromium/public/WebCompositor.h"
 #include "third_party/WebKit/Source/WebKit/chromium/public/WebFloatPoint.h"
@@ skipping 156 matching lines @@
 void CompositorCC::OnNotifyEnd() {
 }
 
 void CompositorCC::OnWidgetSizeChanged() {
   host_.setViewportSize(size());
   root_web_layer_.setBounds(size());
 }
 
 void CompositorCC::OnRootLayerChanged() {
   root_web_layer_.removeAllChildren();
-  root_web_layer_.addChild(root_layer()->web_layer());
+  if (root_layer())
+    root_web_layer_.addChild(root_layer()->web_layer());
 }
 
 void CompositorCC::DrawTree() {
   host_.composite();
 }
 
 bool CompositorCC::ReadPixels(SkBitmap* bitmap) {
   bitmap->setConfig(SkBitmap::kARGB_8888_Config,
                     size().width(), size().height());
   bitmap->allocPixels();
@@ skipping 37 matching lines @@
   ScheduleDraw();
 }
 
 Compositor* Compositor::Create(CompositorDelegate* owner,
                                gfx::AcceleratedWidget widget,
                                const gfx::Size& size) {
   return new CompositorCC(owner, widget, size);
 }
 
 }  // namespace ui