Presentation API: implement .getAvalability() (Blink side).

Source/modules/presentation/Presentation.idl

 // Copyright 2014 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/presentation-api/#navigatorpresentation
 
 [
     GarbageCollected,
     RuntimeEnabled=Presentation
 ] interface Presentation : EventTarget {
     readonly attribute PresentationSession? session;
 
     [CallWith=ScriptState] Promise<PresentationSession> startSession(DOMString presentationUrl, [Default=Undefined] optional DOMString presentationId);
     [CallWith=ScriptState] Promise<PresentationSession> joinSession(DOMString presentationUrl, [Default=Undefined] optional DOMString presentationId);
 
-    attribute EventHandler onavailablechange;
     attribute EventHandler ondefaultsessionstart;
+
+    [CallWith=ScriptState] Promise<PresentationAvailability> getAvailability(DOMString presentationUrl);
 };