content: Cleanup class/struct forward declarations

content/browser/tracing/background_tracing_manager_impl.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 CONTENT_BROWSER_TRACING_BACKGROUND_TRACING_MANAGER_IMPL_H_
 #define CONTENT_BROWSER_TRACING_BACKGROUND_TRACING_MANAGER_IMPL_H_
 
 #include <memory>
 
 #include "base/lazy_instance.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted_memory.h"
 #include "base/memory/weak_ptr.h"
 #include "base/metrics/histogram.h"
 #include "content/browser/tracing/background_tracing_config_impl.h"
 #include "content/browser/tracing/tracing_controller_impl.h"
 #include "content/public/browser/background_tracing_manager.h"
 
 namespace content {
 
 class BackgroundTracingRule;
-class TraceMessageFilter;
 class TracingDelegate;
 
 class BackgroundTracingManagerImpl : public BackgroundTracingManager {
  public:
   static CONTENT_EXPORT BackgroundTracingManagerImpl* GetInstance();
 
   bool SetActiveScenario(std::unique_ptr<BackgroundTracingConfig>,
                          const ReceiveCallback&,
                          DataFiltering data_filtering) override;
   void WhenIdle(IdleCallback idle_callback) override;
@@ skipping 76 matching lines @@
   base::Closure rule_triggered_callback_for_testing_;
 
   friend struct base::DefaultLazyInstanceTraits<BackgroundTracingManagerImpl>;
 
   DISALLOW_COPY_AND_ASSIGN(BackgroundTracingManagerImpl);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_TRACING_BACKGROUND_TRACING_MANAGER_IMPL_H_