Add PrerenderLocalPredictor, which will eventually perform

chrome/browser/history/history.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 874 matching lines @@
     history::TopSites* ts = profile_->GetTopSitesWithoutCreating();
     if (ts)
       ts->MigrateFromHistory();
   }
 }
 
 void HistoryService::OnTopSitesReady() {
   ScheduleAndForget(PRIORITY_NORMAL,
                     &HistoryBackend::MigrateThumbnailsDatabase);
 }
+
+void HistoryService::AddVisitDatabaseObserver(
+    history::VisitDatabaseObserver* observer) {
+  LoadBackendIfNecessary();
+  history_backend_->AddVisitDatabaseObserver(observer);
+}
+
+void HistoryService::RemoveVisitDatabaseObserver(
+    history::VisitDatabaseObserver* observer) {
+  // If the history backend is no longer present, nothing to do here.
+  if (history_backend_)
+    history_backend_->RemoveVisitDatabaseObserver(observer);
+}