aura: Ensure OwnedMailbox not used with invalid or destroyed GLHelper.

content/browser/aura/owned_mailbox.h

 // Copyright 2013 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 "base/memory/ref_counted.h"
 #include "content/browser/aura/image_transport_factory.h"
 #include "gpu/command_buffer/common/mailbox.h"
 
 namespace content {
 
 class GLHelper;
 
 // This class holds a texture id and gpu::Mailbox, and deletes the texture
 // id when the object itself is destroyed. Should only be created if a GLHelper
 // exists on the ImageTransportFactory.
 class OwnedMailbox : public base::RefCounted<OwnedMailbox>,
                      public ImageTransportFactoryObserver {
  public:
   explicit OwnedMailbox(GLHelper* gl_helper);
 
   uint32 texture_id() const { return texture_id_; }
   uint32 sync_point() const { return sync_point_; }
   const gpu::Mailbox& mailbox() const { return mailbox_; }
 
   void UpdateSyncPoint(uint32 sync_point);
+  void Destroy();
 
  protected:
   virtual ~OwnedMailbox();
 
+  // ImageTransportFactoryObserver implementation.
   virtual void OnLostResources() OVERRIDE;
 
  private:
   friend class base::RefCounted<OwnedMailbox>;
 
   uint32 texture_id_;
   gpu::Mailbox mailbox_;
   uint32 sync_point_;
   GLHelper* gl_helper_;
 };
 
 }  // namespace content