components: Cleanup class/struct fwd declarations

components/sync/syncable/directory_backing_store.h

 // Copyright 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.
 
 #ifndef COMPONENTS_SYNC_SYNCABLE_DIRECTORY_BACKING_STORE_H_
 #define COMPONENTS_SYNC_SYNCABLE_DIRECTORY_BACKING_STORE_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 18 matching lines @@
 namespace sync_pb {
 class EntitySpecifics;
 }
 
 namespace syncer {
 namespace syncable {
 
 extern const int32_t kCurrentDBVersion;
 extern const int32_t kCurrentPageSizeKB;
 
-struct ColumnSpec;
-
 // Interface that provides persistence for a syncable::Directory object. You can
 // load all the persisted data to prime a syncable::Directory on startup by
 // invoking Load.  The only other thing you (or more correctly, a Directory) can
 // do here is save any changes that have occurred since calling Load, which can
 // be done periodically as often as desired.
 //
 // The DirectoryBackingStore will own an sqlite lock on its database for most of
 // its lifetime.  You must not have two DirectoryBackingStore objects accessing
 // the database simultaneously.  Because the lock exists at the database level,
 // not even two separate browser instances would be able to acquire it
@@ skipping 218 matching lines @@
   // sql::Connection is destroyed/recreated.
   base::Closure catastrophic_error_handler_;
 
   DISALLOW_COPY_AND_ASSIGN(DirectoryBackingStore);
 };
 
 }  // namespace syncable
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_SYNCABLE_DIRECTORY_BACKING_STORE_H_