Allow WebExternalTextureLayerClient to work with mailboxes.

cc/layers/texture_layer_client.h

 // Copyright 2012 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 CC_LAYERS_TEXTURE_LAYER_CLIENT_H_
 #define CC_LAYERS_TEXTURE_LAYER_CLIENT_H_
 
 namespace WebKit { class WebGraphicsContext3D; }
 
 namespace cc {
 class ResourceUpdateQueue;
+class TextureMailbox;
 
 class TextureLayerClient {
  public:
   // Called to prepare this layer's texture for compositing. The client may
   // queue a texture upload or copy on the ResourceUpdateQueue.
   // Returns the texture ID to be used for compositing.
   virtual unsigned PrepareTexture(ResourceUpdateQueue* queue) = 0;
 
   // Returns the context that is providing the texture. Used for rate limiting
   // and detecting lost context.
   virtual WebKit::WebGraphicsContext3D* Context3d() = 0;
 
+  // Returns true and provides a mailbox if a new frame is available.
+  // Returns false if no new data is available
+  // and the old mailbox is to be reused.
+  virtual bool PrepareTextureMailbox(TextureMailbox* mailbox) = 0;
+
  protected:
   virtual ~TextureLayerClient() {}
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_TEXTURE_LAYER_CLIENT_H_