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

Source/modules/presentation/Presentation.h

Index: Source/modules/presentation/Presentation.h
diff --git a/Source/modules/presentation/Presentation.h b/Source/modules/presentation/Presentation.h
index b324a562654628f786f10582bbd7715684de7fe2..8a5ce56b7ffacd79e69c2b1c3bbe3ed1e235e886 100644
--- a/Source/modules/presentation/Presentation.h
+++ b/Source/modules/presentation/Presentation.h
@@ -46,6 +46,7 @@ public:
     ScriptPromise joinSession(ScriptState*, const String& presentationUrl, const String& presentationId);
 
     DEFINE_ATTRIBUTE_EVENT_LISTENER(availablechange);
+    DEFINE_ATTRIBUTE_EVENT_LISTENER(defaultsessionstart);
 
     // The embedder needs to keep track if anything is listening to the event so it could stop the
     // might be expensive screen discovery process.
@@ -57,6 +58,10 @@ public:
     void didChangeAvailability(bool available);
     // Queried by the controller if |availablechange| event has any listeners.
     bool isAvailableChangeWatched() const;
+
+    // Called when the |defaultsessionstart| event needs to be fired.
+    void didStartDefaultSession(PresentationSession*);
+
     // Adds a session to the open sessions list.
     void registerSession(PresentationSession*);