Introduce the Background Fetch module.

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

Index: third_party/WebKit/Source/modules/background_fetch/BackgroundFetchManager.idl
diff --git a/third_party/WebKit/Source/modules/background_fetch/BackgroundFetchManager.idl b/third_party/WebKit/Source/modules/background_fetch/BackgroundFetchManager.idl
new file mode 100644
index 0000000000000000000000000000000000000000..18b02a808af920da8856d3031a9130eb224db0e1
--- /dev/null
+++ b/third_party/WebKit/Source/modules/background_fetch/BackgroundFetchManager.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-manager
+
+[
+    Exposed=(Window,Worker),
+    RuntimeEnabled=BackgroundFetch
+] interface BackgroundFetchManager {
+    // TODO(peter): Implement the `fetch` method.
+    // TODO(peter): Implement the `get` method.
+
+    [CallWith=ScriptState] Promise<FrozenArray<DOMString>> getTags();
+};