Cleanup: Remove forward declarations in various chrome/browser directories.

chrome/browser/history/in_memory_history_backend.h

 // Copyright (c) 2011 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.
 
 // Contains the history backend wrapper around the in-memory URL database. This
 // object maintains an in-memory cache of the subset of history required to do
 // in-line autocomplete.
 //
 // It is created on the history thread and passed to the main thread where
 // operations can be completed synchronously. It listens for notifications
 // from the "regular" history backend and keeps itself in sync.
 
 #ifndef CHROME_BROWSER_HISTORY_IN_MEMORY_HISTORY_BACKEND_H_
 #define CHROME_BROWSER_HISTORY_IN_MEMORY_HISTORY_BACKEND_H_
 #pragma once
 
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/gtest_prod_util.h"
 #include "base/memory/scoped_ptr.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
 
 class FilePath;
 class GURL;
-class HistoryDatabase;
 class Profile;
 
 namespace history {
 
 class InMemoryDatabase;
 class InMemoryURLIndex;
 struct KeywordSearchTermDetails;
 class URLDatabase;
 struct URLsDeletedDetails;
 struct URLsModifiedDetails;
@@ skipping 60 matching lines @@
 
   // The index used for quick history lookups.
   scoped_ptr<history::InMemoryURLIndex> index_;
 
   DISALLOW_COPY_AND_ASSIGN(InMemoryHistoryBackend);
 };
 
 }  // namespace history
 
 #endif  // CHROME_BROWSER_HISTORY_IN_MEMORY_HISTORY_BACKEND_H_