[Blink, RemotePlayback] watchAvailability() implementation.

third_party/WebKit/Source/modules/remoteplayback/RemotePlayback.idl

 // Copyright 2016 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/remote-playback/#idl-def-remoteplayback
 
 enum RemotePlaybackState {
     "connected",
     "disconnected"
 };
 
+callback RemotePlaybackAvailabilityCallback = void(boolean available);
+
 [
     ActiveScriptWrappable,
+    Custom=VisitDOMWrapper,
     DependentLifetime,
     RuntimeEnabled=RemotePlayback
 ] interface RemotePlayback : EventTarget {
     readonly attribute RemotePlaybackState state;
     attribute EventHandler onconnecting;
     attribute EventHandler onconnect;
     attribute EventHandler ondisconnect;
 
-    [CallWith=ScriptState] Promise<RemotePlaybackAvailability> getAvailability();
-    [CallWith=ScriptState] Promise<void> prompt();
+    Promise<long> watchAvailability(RemotePlaybackAvailabilityCallback callback);
+    Promise<void> cancelWatchAvailability(optional long id);
+    Promise<void> prompt();
 };