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

chrome/browser/history/history_backend.h

Index: chrome/browser/history/history_backend.h
diff --git a/chrome/browser/history/history_backend.h b/chrome/browser/history/history_backend.h
index e5a75304954be88026e5489780fc87fe554f08dd..68765daaa64c275f7f92a1c6bba4154276c4fef2 100644
--- a/chrome/browser/history/history_backend.h
+++ b/chrome/browser/history/history_backend.h
@@ -28,6 +28,7 @@
 
 class BookmarkService;
 class TestingProfile;
+class TypedUrlSyncableService;
 struct ThumbnailScore;
 
 namespace history {
@@ -429,6 +430,10 @@ class HistoryBackend : public base::RefCountedThreadSafe<HistoryBackend>,
 
   virtual bool GetURL(const GURL& url, history::URLRow* url_row);
 
+  // Returns the syncable service for syncing typed urls. The returned service
+  // is owned by |this| object.
+  virtual TypedUrlSyncableService* GetTypedUrlSyncableService() const;
+
   // Deleting ------------------------------------------------------------------
 
   virtual void DeleteURLs(const std::vector<GURL>& urls);
@@ -895,6 +900,9 @@ class HistoryBackend : public base::RefCountedThreadSafe<HistoryBackend>,
   scoped_ptr<AndroidProviderBackend> android_provider_backend_;
 #endif
 
+  // Used to sync typed urls

[brettw, 2013/04/20 05:14:18]

This comment seems redundant with the variable type. Maybe replace it with a
quick sentence about whether/when this can be null?

+  scoped_ptr<TypedUrlSyncableService> typed_url_syncable_service_;
+
   DISALLOW_COPY_AND_ASSIGN(HistoryBackend);
 };