Implement BackgroundFetchManager::{get, fetch}()

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

 // 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-manager
 
 [
     Exposed=(Window,Worker),
     RuntimeEnabled=BackgroundFetch
 ] interface BackgroundFetchManager {
-    // TODO(peter): Implement the `fetch` method.
-    // TODO(peter): Implement the `get` method.
+    // TODO(peter): The `requests` argument of `fetch()` should accept either an
+    // individual RequestInfo, or a sequence of them.
+    [CallWith=ScriptState] Promise<BackgroundFetchRegistration> fetch(DOMString tag, sequence<RequestInfo> requests, optional BackgroundFetchOptions options);
 
+    [CallWith=ScriptState] Promise<BackgroundFetchRegistration?> get(DOMString tag);
     [CallWith=ScriptState] Promise<FrozenArray<DOMString>> getTags();
 };