Implement the events for Background Fetch

third_party/WebKit/Source/modules/background_fetch/BackgroundFetchedEvent.idl

Index: third_party/WebKit/Source/modules/background_fetch/BackgroundFetchedEvent.idl
diff --git a/third_party/WebKit/Source/modules/background_fetch/BackgroundFetchedEvent.idl b/third_party/WebKit/Source/modules/background_fetch/BackgroundFetchedEvent.idl
new file mode 100644
index 0000000000000000000000000000000000000000..8f58cb3bca13b5fb0574fb270cdb1aee2450aa18
--- /dev/null
+++ b/third_party/WebKit/Source/modules/background_fetch/BackgroundFetchedEvent.idl
@@ -0,0 +1,15 @@
+// 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.
+
+// https://wicg.github.io/background-fetch/#background-fetch-end-event
+
+[
+    Constructor(DOMString type, BackgroundFetchedEventInit init),
+    Exposed=ServiceWorker,
+    RuntimeEnabled=BackgroundFetch
+] interface BackgroundFetchedEvent : BackgroundFetchEvent {
+    // TODO(peter): Implement the `completeFetches` attribute.
+
+    [CallWith=ScriptState] Promise<void> updateUI(DOMString title);
+};