CleanUp : Remove unused ReadbackRequest variable.

cc/trees/thread_proxy.h

 // Copyright 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 CC_TREES_THREAD_PROXY_H_
 #define CC_TREES_THREAD_PROXY_H_
 
 #include <string>
 
 #include "base/memory/scoped_ptr.h"
@@ skipping 76 matching lines @@
 
     PrioritizedResourceManager* contents_texture_manager();
 
     LayerTreeHost* layer_tree_host;
     bool commit_waits_for_activation;
     bool main_thread_inside_commit;
 
     base::TimeTicks last_monotonic_frame_begin_time;
   };
 
-  struct ReadbackRequest;
-
   struct CompositorThreadOnly {
     CompositorThreadOnly(
         ThreadProxy* proxy,
         int layer_tree_host_id,
         RenderingStatsInstrumentation* rendering_stats_instrumentation);
     ~CompositorThreadOnly();
 
     const int layer_tree_host_id;
 
     // Copy of the main thread side contents texture manager for work
@@ skipping 193 matching lines @@
 
   base::WeakPtr<ThreadProxy> main_thread_weak_ptr_;
   base::WeakPtr<ThreadProxy> impl_thread_weak_ptr_;
 
   DISALLOW_COPY_AND_ASSIGN(ThreadProxy);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_THREAD_PROXY_H_