[PresentationAPI] Plumbing |onstatechange| event for the PresentationSession from Blink to platform/

public/platform/modules/presentation/WebPresentationSessionClient.h

 // 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.
 
 #ifndef WebPresentationSessionClient_h
 #define WebPresentationSessionClient_h
 
 #include "public/platform/WebString.h"
 
 namespace blink {
 
+enum class WebPresentationSessionState {
+    Connected = 0,
+    Disconnected,
+};
+
 // The implementation the embedder has to provide for the Presentation API to work.
 class WebPresentationSessionClient {
 public:
     virtual ~WebPresentationSessionClient() { }
 
     virtual WebString getId() = 0;
     virtual WebString getUrl() = 0;
 };
 
 } // namespace blink
 
 #endif // WebPresentationSessionClient_h