[Invalidations] Require there to be no registered handlers on Invalidator destruction

sync/notifier/invalidator.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 invalidator, which is an object that receives
 // invalidations for registered object IDs. The corresponding
 // InvalidationHandler is notifier when such an event occurs.
 
 #ifndef SYNC_NOTIFIER_INVALIDATOR_H_
 #define SYNC_NOTIFIER_INVALIDATOR_H_
@@ skipping 30 matching lines @@
   //   invalidator->UnregisterHandler(client_handler);
   //
   // Note that there's no call to UpdateRegisteredIds() -- this is because the
   // invalidation API persists registrations across browser restarts.
   //
   // When permanently shutting down the client, e.g. when disabling the related
   // feature:
   //
   //   invalidator->UpdateRegisteredIds(client_handler, ObjectIdSet());
   //   invalidator->UnregisterHandler(client_handler);
+  //
+  // 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;
@@ skipping 27 matching lines @@
   // 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 SendInvalidation(
       const ObjectIdInvalidationMap& invalidation_map) = 0;
 };
 }  // namespace syncer
 
 #endif  // SYNC_NOTIFIER_INVALIDATOR_H_