Add IDL and initial Blink API for Background Sync

Source/modules/background_sync/SyncRegistrationOptions.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.
+
+enum SyncNetworkType {
+  "network-any",
+  "network-offline",
+  "network-online",
+  "network-non-mobile",
+};
+
+[
+    RuntimeEnabled=BackgroundSync,
+] dictionary SyncRegistrationOptions {
+    DOMString id;
+    unsigned long minDelay = 0;
+    unsigned long maxDelay = 0;
+    unsigned long minPeriod = 0;
+    SyncNetworkType minRequiredNetwork = "network-online";
+    boolean allowOnBattery = true;
+    boolean idleRequired = false;
+};