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

Source/modules/presentation/PresentationSession.h

Index: Source/modules/presentation/PresentationSession.h
diff --git a/Source/modules/presentation/PresentationSession.h b/Source/modules/presentation/PresentationSession.h
index fccd2ee783d40848e5045f0bee620505e0f6aa6e..378995d434d77e45b804057c0fda35fbafcf9afd 100644
--- a/Source/modules/presentation/PresentationSession.h
+++ b/Source/modules/presentation/PresentationSession.h
@@ -55,6 +55,9 @@ public:
     // Notifies the session about its state change.
     void didChangeState(WebPresentationSessionState);
 
+    // Notifies the session about new text message.
+    void didReceiveTextMessage(const String& message);
+
 private:
     PresentationSession(LocalFrame*, const String& id, const String& url);