[Sync] Enable adding notifier observers from ProfileSyncService

sync/notifier/sync_notifier.h

 // Copyright (c) 2012 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 sync notifier, which is an object that receives
 // notifications when updates are available for a set of sync types.
 // All the observers are notified when such an event happens.
 
 #ifndef SYNC_NOTIFIER_SYNC_NOTIFIER_H_
 #define SYNC_NOTIFIER_SYNC_NOTIFIER_H_
 
 #include <string>
 
 #include "sync/internal_api/public/base/model_type.h"
 #include "sync/notifier/invalidation_util.h"
 
 namespace syncer {
 class SyncNotifierObserver;
 
 class SyncNotifier {
  public:
   SyncNotifier() {}
   virtual ~SyncNotifier() {}
 
-  // Updates the set of ObjectIds associated with a given |handler|. Passing an
-  // empty ObjectIdSet will unregister |handler|. If two different handlers
-  // attempt to register for the same object ID, the first registration wins.
+  // Updates the set of ObjectIds associated with a given

[msw, 2012/08/03 05:14:21]

nit: consider the same line breaking that I suggested earlier.

+  // |handler|. Passing an empty ObjectIdSet will unregister
+  // |handler|. There should be at most one handler registered per
+  // object id.
   virtual void UpdateRegisteredIds(SyncNotifierObserver* handler,
                                    const ObjectIdSet& ids) = 0;
 
   // SetUniqueId must be called once, before any call to
   // UpdateCredentials.  |unique_id| should be a non-empty globally
   // unique string.
   virtual void SetUniqueId(const std::string& unique_id) = 0;
 
   // SetState must be called once, before any call to
   // UpdateCredentials.  |state| may be empty.
   // Deprecated in favour of InvalidationStateTracker persistence.
   virtual void SetStateDeprecated(const std::string& state) = 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;
 
   // This is here only to support the old p2p notification implementation,
   // which is still used by sync integration tests.
   // TODO(akalin): Remove this once we move the integration tests off p2p
   // notifications.
   virtual void SendNotification(ModelTypeSet changed_types) = 0;
 };
 }  // namespace syncer
 
 #endif  // SYNC_NOTIFIER_SYNC_NOTIFIER_H_