[Sync] InvalidationService shouldn't CHECK when registering ObjectId for more than one handler

components/invalidation/invalidator_registrar.h

 // Copyright 2014 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.
 
 #ifndef COMPONENTS_INVALIDATION_INVALIDATOR_REGISTRAR_H_
 #define COMPONENTS_INVALIDATION_INVALIDATOR_REGISTRAR_H_
 
 #include <map>
 
 #include "base/basictypes.h"
@@ skipping 21 matching lines @@
 
   // It is an error to have registered handlers on destruction.
   ~InvalidatorRegistrar();
 
   // Starts sending notifications to |handler|.  |handler| must not be NULL,
   // and it must already be registered.
   void RegisterHandler(InvalidationHandler* handler);
 
   // Updates the set of ObjectIds associated with |handler|.  |handler| must
   // not be NULL, and must already be registered.  An ID must be registered for
-  // at most one handler.
-  void UpdateRegisteredIds(InvalidationHandler* handler,
-                           const ObjectIdSet& ids);
+  // at most one handler. If ID is already registered function returns false.
+  bool UpdateRegisteredIds(InvalidationHandler* handler,
+                           const ObjectIdSet& ids) WARN_UNUSED_RESULT;
 
   // Stops sending notifications to |handler|.  |handler| must not be NULL, and
   // it must already be registered.  Note that this doesn't unregister the IDs
   // associated with |handler|.
   void UnregisterHandler(InvalidationHandler* handler);
 
   ObjectIdSet GetRegisteredIds(InvalidationHandler* handler) const;
 
   // Returns the set of all IDs that are registered to some handler (even
   // handlers that have been unregistered).
@@ skipping 33 matching lines @@
   ObserverList<InvalidationHandler, true> handlers_;
   HandlerIdsMap handler_to_ids_map_;
   InvalidatorState state_;
 
   DISALLOW_COPY_AND_ASSIGN(InvalidatorRegistrar);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_INVALIDATION_INVALIDATOR_REGISTRAR_H_