BackgroundFetchRegistration.abort() should return a Promise.

third_party/WebKit/public/platform/modules/background_fetch/background_fetch.mojom

 // Copyright 2017 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.
 
 module blink.mojom;
 
 enum BackgroundFetchError {
   NONE,
-  DUPLICATED_TAG
+  DUPLICATED_TAG,
+  INVALID_TAG
 };
 
 struct IconDefinition {
   string src;
   string sizes;
   string type;
 };
 
 struct BackgroundFetchRegistration {
   string tag;
   array<IconDefinition> icons;
   int64 total_download_size = 0;
   string title = "";
 };
 
 interface BackgroundFetchService {
   // TODO(peter): Implement support for the `fetch()` function in Mojo.
 
   // Updates the user interface for the Background Fetch identified by the
   // |service_worker_registration_id| and the |tag|.
   UpdateUI(int64 service_worker_registration_id, string tag, string title)
       => (BackgroundFetchError error);
 
   // Aborts the Background Fetch registration identified by the
   // |service_worker_registration_id| and the |tag|.
-  Abort(int64 service_worker_registration_id, string tag);
+  Abort(int64 service_worker_registration_id, string tag)
+      => (BackgroundFetchError error);
 
   // Gets the Background Fetch registration identified by the
   // |service_worker_registration_id| and the |tag|.
   GetRegistration(int64 service_worker_registration_id, string tag)
       => (BackgroundFetchError error,
           BackgroundFetchRegistration? registration);
 
   // Gets the sequence of tags for active Background Fetch registrations given
   // the |service_worker_registration_id|.
   GetTags(int64 service_worker_registration_id)
       => (BackgroundFetchError error,
           array<string> tags);
 };