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

public/platform/modules/presentation/WebPresentationController.h

Index: public/platform/modules/presentation/WebPresentationController.h
diff --git a/public/platform/modules/presentation/WebPresentationController.h b/public/platform/modules/presentation/WebPresentationController.h
index 9ce4747794957b3ec0ffd743e138282fb5ee0083..6d4a393483a933de8e3fa9c434737734475a0c1b 100644
--- a/public/platform/modules/presentation/WebPresentationController.h
+++ b/public/platform/modules/presentation/WebPresentationController.h
@@ -7,6 +7,8 @@
 
 namespace blink {
 
+class WebPresentationSessionClient;
+
 // The delegate Blink provides to WebPresentationClient in order to get updates.
 class WebPresentationController {
 public:
@@ -18,6 +20,10 @@ public:
 
     // Indicates if the frame has listeners to the |availablechange| event.
     virtual bool isAvailableChangeWatched() const = 0;
+
+    // Called when the default presentation is started by the embedder.
+    // May not be called if there's no registered listener for the event.
+    virtual void didStartDefaultPresentation(WebPresentationSessionClient*) = 0;
 };
 
 } // namespace blink