Use WebCompositor only when --enable-threaded-compositing.

content/renderer/render_thread_impl.h

 // 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.
 
 #ifndef CONTENT_RENDERER_RENDER_THREAD_IMPL_H_
 #define CONTENT_RENDERER_RENDER_THREAD_IMPL_H_
 #pragma once
 
 #include <set>
 #include <string>
@@ skipping 116 matching lines @@
 #endif
 
   // These methods modify how the next message is sent.  Normally, when sending
   // a synchronous message that runs a nested message loop, we need to suspend
   // callbacks into WebKit.  This involves disabling timers and deferring
   // resource loads.  However, there are exceptions when we need to customize
   // the behavior.
   void DoNotSuspendWebKitSharedTimer();
   void DoNotNotifyWebKitOfModalLoop();
 
+  // Will be NULL if threaded compositing has not been enabled.
   CompositorThread* compositor_thread() const {
     return compositor_thread_.get();
   }
 
   AppCacheDispatcher* appcache_dispatcher() const {
     return appcache_dispatcher_.get();
   }
 
   IndexedDBDispatcher* indexed_db_dispatcher() const {
     return indexed_db_dispatcher_.get();
@@ skipping 90 matching lines @@
   std::set<std::string> v8_extensions_;
 
   scoped_ptr<CompositorThread> compositor_thread_;
 
   ObserverList<content::RenderProcessObserver> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(RenderThreadImpl);
 };
 
 #endif  // CONTENT_RENDERER_RENDER_THREAD_IMPL_H_