BackgroundFetchRegistration.abort() should return a Promise.

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

Index: third_party/WebKit/public/platform/modules/background_fetch/background_fetch.mojom
diff --git a/third_party/WebKit/public/platform/modules/background_fetch/background_fetch.mojom b/third_party/WebKit/public/platform/modules/background_fetch/background_fetch.mojom
index 9092281d0e4a846e2171735d1dc7850af980e3be..381a3c889f31a4bb87c4d0d2f816604d6a653bc6 100644
--- a/third_party/WebKit/public/platform/modules/background_fetch/background_fetch.mojom
+++ b/third_party/WebKit/public/platform/modules/background_fetch/background_fetch.mojom
@@ -6,7 +6,8 @@ module blink.mojom;
 
 enum BackgroundFetchError {
   NONE,
-  DUPLICATED_TAG
+  DUPLICATED_TAG,
+  INVALID_TAG
 };
 
 struct IconDefinition {
@@ -32,7 +33,8 @@ interface BackgroundFetchService {
 
   // 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|.