[PresentationAPI] Added on-session-text-message handler.

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 b703f5646a9c0fd6a71ddeaa7a37f578ab79877f..e74ece3bebc07c7199effdd0819b1ea59de9932a 100644
--- a/Source/modules/presentation/Presentation.h
+++ b/Source/modules/presentation/Presentation.h
@@ -67,6 +67,9 @@ public:
     // Called when the |onstatechange| event needs to be fired to the right session.
     void didChangeSessionState(WebPresentationSessionClient*, WebPresentationSessionState);
 
+    // Called when the |onmessage| event needs to be fired to the right session.
+    void didReceiveSessionTextMessage(WebPresentationSessionClient*, const String& message);
+
     // Adds a session to the open sessions list.
     void registerSession(PresentationSession*);