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

sync/notifier/invalidator_registrar.cc

 // 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.
 
 #include "sync/notifier/invalidator_registrar.h"
 
 #include <cstddef>
 #include <utility>
 
 #include "base/logging.h"
 
 namespace syncer {
 
 InvalidatorRegistrar::InvalidatorRegistrar()
     : state_(DEFAULT_INVALIDATION_ERROR) {}
 
 InvalidatorRegistrar::~InvalidatorRegistrar() {
   DCHECK(thread_checker_.CalledOnValidThread());
+  CHECK(!handlers_.might_have_observers());
+  // |id_to_handler_map_| may be non-empty but that's okay.
 }
 
 void InvalidatorRegistrar::RegisterHandler(InvalidationHandler* handler) {
   DCHECK(thread_checker_.CalledOnValidThread());
   CHECK(handler);
   CHECK(!handlers_.HasObserver(handler));
   handlers_.AddObserver(handler);
 }
 
 void InvalidatorRegistrar::UpdateRegisteredIds(
@@ skipping 111 matching lines @@
 }
 
 InvalidationHandler* InvalidatorRegistrar::ObjectIdToHandler(
     const invalidation::ObjectId& id) {
   DCHECK(thread_checker_.CalledOnValidThread());
   IdHandlerMap::const_iterator it = id_to_handler_map_.find(id);
   return (it == id_to_handler_map_.end()) ? NULL : it->second;
 }
 
 }  // namespace syncer