[Sync] Add interface and backend impl for typed URL syncable service

chrome/browser/history/history_service.cc

 // 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 history system runs on a background thread so that potentially slow
 // database operations don't delay the browser. This backend processing is
 // represented by HistoryBackend. The HistoryService's job is to dispatch to
 // that thread.
 //
 // Main thread                       History thread
@@ skipping 344 matching lines @@
 
 bool HistoryService::GetVisitCountForURL(const GURL& url, int* visit_count) {
   DCHECK(thread_checker_.CalledOnValidThread());
   history::URLRow url_row;
   if (!GetRowForURL(url, &url_row))
     return false;
   *visit_count = url_row.visit_count();
   return true;
 }
 
+history::TypedUrlSyncableService* HistoryService::GetTypedUrlSyncableService()
+    const {
+  return history_backend_->GetTypedUrlSyncableService();
+}
+
 void HistoryService::Shutdown() {
   DCHECK(thread_checker_.CalledOnValidThread());
   // It's possible that bookmarks haven't loaded and history is waiting for
   // bookmarks to complete loading. In such a situation history can't shutdown
   // (meaning if we invoked history_service_->Cleanup now, we would
   // deadlock). To break the deadlock we tell BookmarkModel it's about to be
   // deleted so that it can release the signal history is waiting on, allowing
   // history to shutdown (history_service_->Cleanup to complete). In such a
   // scenario history sees an incorrect view of bookmarks, but it's better
   // than a deadlock.
@@ skipping 890 matching lines @@
   DCHECK(thread_checker_.CalledOnValidThread());
   visit_database_observers_.RemoveObserver(observer);
 }
 
 void HistoryService::NotifyVisitDBObserversOnAddVisit(
     const history::BriefVisitInfo& info) {
   DCHECK(thread_checker_.CalledOnValidThread());
   FOR_EACH_OBSERVER(history::VisitDatabaseObserver, visit_database_observers_,
                     OnAddVisit(info));
 }