components: Cleanup class/struct fwd declarations

components/history/core/browser/history_client.h

 // Copyright 2014 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 COMPONENTS_HISTORY_CORE_BROWSER_HISTORY_CLIENT_H_
 #define COMPONENTS_HISTORY_CORE_BROWSER_HISTORY_CLIENT_H_
 
 #include <memory>
 
 #include "base/macros.h"
 #include "sql/init_status.h"
 
 class GURL;
 
-namespace base {
-class FilePath;
-}
-
 namespace history {
 
-class HistoryBackend;
 class HistoryBackendClient;
-class HistoryDatabase;
 class HistoryService;
-class ThumbnailDatabase;
 
 // This class abstracts operations that depend on the embedder's environment,
 // e.g. Chrome.
 class HistoryClient {
  public:
   HistoryClient() {}
   virtual ~HistoryClient() {}
 
   // Called upon HistoryService creation.
   virtual void OnHistoryServiceCreated(HistoryService* history_service) = 0;
@@ skipping 12 matching lines @@
   // Returns a new HistoryBackendClient instance.
   virtual std::unique_ptr<HistoryBackendClient> CreateBackendClient() = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(HistoryClient);
 };
 
 }  // namespace history
 
 #endif  // COMPONENTS_HISTORY_CORE_BROWSER_HISTORY_CLIENT_H_