cc: Cleanup class/struct forward declarations

cc/trees/proxy_common.h

 // Copyright 2015 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_PROXY_COMMON_H_
 #define CC_TREES_PROXY_COMMON_H_
 
 #include <stddef.h>
 
 #include "base/callback_forward.h"
 #include "cc/base/cc_export.h"
 #include "cc/output/begin_frame_args.h"
 #include "cc/trees/layer_tree_host_common.h"
 
 namespace cc {
 
-class LayerTreeHost;
-
 using BeginFrameCallbackList = std::vector<base::Closure>;
 
 struct CC_EXPORT BeginMainFrameAndCommitState {
   BeginMainFrameAndCommitState();
   ~BeginMainFrameAndCommitState();
 
   unsigned int begin_frame_id = 0;
   BeginFrameArgs begin_frame_args;
   std::unique_ptr<BeginFrameCallbackList> begin_frame_callbacks;
   std::unique_ptr<ScrollAndScaleSet> scroll_info;
   size_t memory_allocation_limit_bytes = 0;
   bool evicted_ui_resources = false;
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_PROXY_COMMON_H_