components: Cleanup class/struct fwd declarations

components/history/core/browser/in_memory_history_backend.h

 // Copyright (c) 2012 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.
 
 // The InMemoryHistoryBackend is a wrapper around the in-memory URL database.
 // It maintains an in-memory cache of a subset of history that is required for
 // low-latency operations, such as in-line autocomplete.
 //
 // The in-memory cache provides the following guarantees:
 //  (1.) It will always contain URLRows that either have a |typed_count| > 0; or
@@ skipping 21 matching lines @@
 namespace base {
 class FilePath;
 }
 
 namespace history {
 
 class HistoryBackendTestBase;
 class HistoryService;
 class InMemoryDatabase;
 class InMemoryHistoryBackendTest;
-class URLDatabase;
 class URLRow;
 
 class InMemoryHistoryBackend : public HistoryServiceObserver {
  public:
   InMemoryHistoryBackend();
   ~InMemoryHistoryBackend() override;
 
   // Initializes the backend from the history database pointed to by the
   // full path in |history_filename|.
   bool Init(const base::FilePath& history_filename);
@@ skipping 44 matching lines @@
 
   ScopedObserver<HistoryService, HistoryServiceObserver>
       history_service_observer_;
 
   DISALLOW_COPY_AND_ASSIGN(InMemoryHistoryBackend);
 };
 
 }  // namespace history
 
 #endif  // COMPONENTS_HISTORY_CORE_BROWSER_IN_MEMORY_HISTORY_BACKEND_H_