Add IDL and initial Blink API for Background Sync

Source/modules/background_sync/SyncRegistration.idl

 // Copyright 2015 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://w3c.github.io/encrypted-media/#idl-def-MediaKeyStatusMap
-
-enum MediaKeyStatus {
-    "usable",
-    "expired",
-    "output-not-allowed",
-    "internal-error"
+[
+    Exposed=(Window,ServiceWorker),
+    GarbageCollected,
+    RuntimeEnabled=BackgroundSync,
+] interface SyncRegistration {
+    readonly attribute DOMString id;
+    readonly attribute SyncRegistrationOptions options;
+    [CallWith=ScriptState] Promise<boolean> unregister();
 };
-
-[
-    GarbageCollected,
-] interface MediaKeyStatusMap {
-    readonly attribute unsigned long size;
-    readonly maplike <BufferSource, MediaKeyStatus>;
-};