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

chrome/browser/history/history_service.cc

Index: chrome/browser/history/history_service.cc
diff --git a/chrome/browser/history/history_service.cc b/chrome/browser/history/history_service.cc
index e38cbcf1a23af739b70d555a482f616341437e23..7b455debfd402a4da7be917d111ec49fe60bc066 100644
--- a/chrome/browser/history/history_service.cc
+++ b/chrome/browser/history/history_service.cc
@@ -362,6 +362,10 @@ bool HistoryService::GetVisitCountForURL(const GURL& url, int* visit_count) {
   return true;
 }
 
+TypedUrlSyncableService* HistoryService::GetTypedUrlSyncableService() const {
+  return history_backend_->GetTypedUrlSyncableService();

[Andrew T Wilson (Slow), 2013/04/09 12:48:30]

HistoryBackend and TypedUrlSyncableService are run on the DB thread. Why is it
OK to handle references here on the UI thread?

+}
+
 void HistoryService::Shutdown() {
   DCHECK(thread_checker_.CalledOnValidThread());
   // It's possible that bookmarks haven't loaded and history is waiting for