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.
+
+[
+    Exposed=(Window,ServiceWorker),
+    GarbageCollected,
+    RuntimeEnabled=BackgroundSync,
+] interface SyncRegistration {
+    readonly attribute DOMString id;
+    readonly attribute unsigned long minDelay;
+    readonly attribute unsigned long maxDelay;
+    readonly attribute unsigned long minPeriod;
+    readonly attribute SyncNetworkType minRequiredNetwork;
+    readonly attribute boolean allowOnBattery;
+    readonly attribute boolean idleRequired;
+    [CallWith=ScriptState] Promise<boolean> unregister();
+};