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

chrome/browser/sync/invalidation_frontend.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.
 
 #ifndef CHROME_BROWSER_SYNC_INVALIDATION_FRONTEND_H_
 #define CHROME_BROWSER_SYNC_INVALIDATION_FRONTEND_H_
 
 #include "sync/notifier/invalidation_util.h"
 
 namespace syncer {
@@ skipping 29 matching lines @@
 //   frontend->UnregisterInvalidationHandler(client_handler);
 //
 // If an invalidation handler cares about the invalidator state, it should also
 // do the following when starting the client:
 //
 //   invalidator_state = frontend->GetInvalidatorState();
 //
 // It can also do the above in OnInvalidatorStateChange(), or it can use the
 // argument to OnInvalidatorStateChange().
 //
+// It is an error to have registered handlers when an
+// InvalidationFrontend is shut down; clients must ensure that they
+// unregister themselves before then. (Depending on the
+// InvalidationFrontend, shutdown may be equivalent to destruction, or
+// a separate function call like Shutdown()).
+//
 // NOTE(akalin): Invalidations that come in during browser shutdown may get
 // dropped.  This won't matter once we have an Acknowledge API, though: see
 // http://crbug.com/78462 and http://crbug.com/124149.
 class InvalidationFrontend {
  public:
   // Starts sending notifications to |handler|.  |handler| must not be NULL,
   // and it must not already be registered.
   //
   // Handler registrations are persisted across restarts of sync.
   virtual void RegisterInvalidationHandler(
@@ skipping 19 matching lines @@
   // Returns the current invalidator state.  When called from within
   // InvalidationHandler::OnInvalidatorStateChange(), this must return
   // the updated state.
   virtual syncer::InvalidatorState GetInvalidatorState() const = 0;
 
  protected:
   virtual ~InvalidationFrontend() { }
 };
 
 #endif  // CHROME_BROWSER_SYNC_INVALIDATION_FRONTEND_H_