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

components/invalidation/invalidator.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.
 //
 // Interface to the invalidator, which is an object that receives
 // invalidations for registered object IDs. The corresponding
 // InvalidationHandler is notifier when such an event occurs.
 
 #ifndef COMPONENTS_INVALIDATION_INVALIDATOR_H_
 #define COMPONENTS_INVALIDATION_INVALIDATOR_H_
@@ skipping 41 matching lines @@
   // It is an error to have registered handlers when an invalidator is
   // destroyed; clients must ensure that they unregister themselves
   // before then.
 
   // Starts sending notifications to |handler|.  |handler| must not be NULL,
   // and it must not already be registered.
   virtual void RegisterHandler(InvalidationHandler* handler) = 0;
 
   // 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.
-  virtual void UpdateRegisteredIds(InvalidationHandler* handler,
-                                   const ObjectIdSet& ids) = 0;
+  // at most one handler. If ID is already registered function returns false.
+  virtual bool UpdateRegisteredIds(InvalidationHandler* handler,
+                                   const ObjectIdSet& ids)
+      WARN_UNUSED_RESULT = 0;
 
   // 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|.
   virtual void UnregisterHandler(InvalidationHandler* handler) = 0;
 
   // Returns the current invalidator state.  When called from within
   // InvalidationHandler::OnInvalidatorStateChange(), this must return
   // the updated state.
   virtual InvalidatorState GetInvalidatorState() const = 0;
 
   // The observers won't be notified of any notifications until
   // UpdateCredentials is called at least once. It can be called more than
   // once.
   virtual void UpdateCredentials(
       const std::string& email, const std::string& token) = 0;
 
   // Requests internal detailed status to be posted back to the callback.
   virtual void RequestDetailedStatus(
       base::Callback<void(const base::DictionaryValue&)> callback) const = 0;
 };
 }  // namespace syncer
 
 #endif  // COMPONENTS_INVALIDATION_INVALIDATOR_H_