[PresentationAPI] Added on-session-message handler.

content/renderer/presentation/presentation_dispatcher.h

Index: content/renderer/presentation/presentation_dispatcher.h
diff --git a/content/renderer/presentation/presentation_dispatcher.h b/content/renderer/presentation/presentation_dispatcher.h
index fb6b4d9a83b02147194dd362b33dc6e1ef65fec0..0f0645295e0c80e47402b7e60278d544a95bf32d 100644
--- a/content/renderer/presentation/presentation_dispatcher.h
+++ b/content/renderer/presentation/presentation_dispatcher.h
@@ -58,6 +58,9 @@ class CONTENT_EXPORT PresentationDispatcher
   void OnSessionStateChange(
       presentation::PresentationSessionInfoPtr session_info,
       presentation::PresentationSessionState session_state);
+  void OnSessionTextMessageReceived(
+      presentation::PresentationSessionInfoPtr session_info,
+      const std::string& message);
 
   void ConnectToPresentationServiceIfNeeded();