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

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 8ed2da2f86fa5916a696978cba54dd8cd75a50ff..aab8fff61bbd94285f022d69ee1eedd27e9af9d1 100644
--- a/public/platform/modules/presentation/WebPresentationController.h
+++ b/public/platform/modules/presentation/WebPresentationController.h
@@ -8,6 +8,7 @@
 namespace blink {
 
 class WebPresentationSessionClient;
+enum class WebPresentationSessionState;
 
 // The delegate Blink provides to WebPresentationClient in order to get updates.
 class WebPresentationController {
@@ -24,6 +25,9 @@ public:
     // Called when the presentation session is started by the embedder using
     // the default presentation URL and id.
     virtual void didStartDefaultSession(WebPresentationSessionClient*) = 0;
+
+    // Called when the state of a session changes.
+    virtual void didChangeSessionState(WebPresentationSessionClient*, WebPresentationSessionState) = 0;
 };
 
 } // namespace blink