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

sync/notifier/invalidator.h

Index: sync/notifier/invalidator.h
diff --git a/sync/notifier/invalidator.h b/sync/notifier/invalidator.h
index cd5c289f970a4e9517299a5cd37c8bab4f6acac7..4aa5d70543a62dbec70ef91997b6d2b2328dda34 100644
--- a/sync/notifier/invalidator.h
+++ b/sync/notifier/invalidator.h
@@ -48,6 +48,10 @@ class Invalidator {
   //
   //   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.