[Presentation API] Added the DefaultPresentationStart event and parsing of the default presentation…

public/platform/modules/presentation/WebPresentationController.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 WebPresentationController_h
 #define WebPresentationController_h
 
 namespace blink {
 
+class WebPresentationSessionClient;
+
 // The delegate Blink provides to WebPresentationClient in order to get updates.
 class WebPresentationController {
 public:
     virtual ~WebPresentationController() { }
 
     // Called when the presentation screen availability changes.
     // May not be called if there's no registered listener for the event.
     virtual void didChangeAvailability(bool available) = 0;
 
     // Indicates if the frame has listeners to the |availablechange| event.
     virtual bool isAvailableChangeWatched() const = 0;
+
+    // Called when the presentation session is started by the embedder using
+    // the default presentation URL and id.
+    virtual void didStartDefaultSession(WebPresentationSessionClient*) = 0;
 };
 
 } // namespace blink
 
 #endif // WebPresentationController_h