Cleanup remaining class/struct fwd declarations

cc/trees/proxy_main.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_MAIN_H_
 #define CC_TREES_PROXY_MAIN_H_
 
 #include "base/macros.h"
 #include "cc/base/cc_export.h"
 #include "cc/input/browser_controls_state.h"
 #include "cc/trees/proxy.h"
 #include "cc/trees/proxy_common.h"
 
 namespace cc {
 
-class BeginFrameSource;
 class MutatorEvents;
 class CompletionEvent;
 class CompositorFrameSink;
 class LayerTreeHostInProcess;
 class LayerTreeMutator;
 class ProxyImpl;
 
 // This class aggregates all interactions that the impl side of the compositor
 // needs to have with the main side.
 // The class is created and lives on the main thread.
@@ skipping 105 matching lines @@
   std::unique_ptr<ProxyImpl> proxy_impl_;
 
   base::WeakPtrFactory<ProxyMain> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ProxyMain);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_PROXY_MAIN_H_