[sync] skeleton implementation of TypedURLSyncBridge

components/history/core/browser/history_backend.h

Index: components/history/core/browser/history_backend.h
diff --git a/components/history/core/browser/history_backend.h b/components/history/core/browser/history_backend.h
index 4747695807f5dff18aeea5385056069059a143df..f96d2182c87b8af64de5d9378c85a24f99beffbe 100644
--- a/components/history/core/browser/history_backend.h
+++ b/components/history/core/browser/history_backend.h
@@ -53,6 +53,7 @@ struct HistoryDatabaseParams;
 class HistoryDBTask;
 class InMemoryHistoryBackend;
 class TypedUrlSyncableService;
+class TypedURLSyncBridge;
 class HistoryBackendHelper;
 class URLDatabase;
 
@@ -400,6 +401,10 @@ class HistoryBackend : public base::RefCountedThreadSafe<HistoryBackend>,
   // is owned by |this| object.
   virtual TypedUrlSyncableService* GetTypedUrlSyncableService() const;
 
+  // Returns the sync bridge for syncing typed urls. The returned service
+  // is owned by |this| object.
+  TypedURLSyncBridge* GetTypedURLSyncBridge() const;
+
   // Deleting ------------------------------------------------------------------
 
   virtual void DeleteURLs(const std::vector<GURL>& urls);
@@ -909,10 +914,12 @@ class HistoryBackend : public base::RefCountedThreadSafe<HistoryBackend>,
   // List of observers
   base::ObserverList<HistoryBackendObserver> observers_;
 
-  // Used to manage syncing of the typed urls datatype. This will be null before
-  // Init is called. Defined after observers_ because it unregisters itself as
-  // observer during destruction.
+  // Used to manage syncing of the typed urls datatype. They will be null before
+  // Init is called, and only one will be instantiated after Init is called
+  // depending on switches::kSyncUSSTypedURL. Defined after observers_ because
+  // it unregisters itself as observer during destruction.
   std::unique_ptr<TypedUrlSyncableService> typed_url_syncable_service_;
+  std::unique_ptr<TypedURLSyncBridge> typed_url_sync_bridge_;
 
   DISALLOW_COPY_AND_ASSIGN(HistoryBackend);
 };