Implement the events for Background Fetch

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

Index: third_party/WebKit/Source/modules/background_fetch/BackgroundFetchEvent.idl
diff --git a/third_party/WebKit/Source/modules/background_fetch/BackgroundFetchEvent.idl b/third_party/WebKit/Source/modules/background_fetch/BackgroundFetchEvent.idl
new file mode 100644
index 0000000000000000000000000000000000000000..3265f3ec1b71fd5d7d8bceb0492aa46935f388f6
--- /dev/null
+++ b/third_party/WebKit/Source/modules/background_fetch/BackgroundFetchEvent.idl
@@ -0,0 +1,13 @@
+// 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-event
+
+[
+    Constructor(DOMString type, BackgroundFetchEventInit init),
+    Exposed=ServiceWorker,
+    RuntimeEnabled=BackgroundFetch
+] interface BackgroundFetchEvent : ExtendableEvent {
+    readonly attribute DOMString tag;
+};