content: Cleanup class/struct forward declarations

content/test/test_background_sync_context.h

 // Copyright 2016 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_TEST_TEST_BACKGROUND_SYNC_CONTEXT_H_
 #define CONTENT_TEST_TEST_BACKGROUND_SYNC_CONTEXT_H_
 
 #include <memory>
 
 #include "base/macros.h"
 #include "content/browser/background_sync/background_sync_context.h"
 
 namespace content {
 
-class TestBackgroundSyncManager;
-
 // A BackgroundSyncContext for use in unit tests, primarily to create a test
 // BackgroundSyncManager.
 class TestBackgroundSyncContext : public BackgroundSyncContext {
  public:
   TestBackgroundSyncContext() = default;
 
  protected:
   ~TestBackgroundSyncContext() override = default;
 
   // BackgroundSyncContextImpl:
   void CreateBackgroundSyncManager(
       scoped_refptr<ServiceWorkerContextWrapper> context) override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(TestBackgroundSyncContext);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_TEST_TEST_BACKGROUND_SYNC_CONTEXT_H_