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

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 bcb31ef67f80b326788b7da274db968a84defd7e..a539e577ae9126a9446c70122995789c9c7a03de 100644
--- a/public/platform/modules/presentation/WebPresentationController.h
+++ b/public/platform/modules/presentation/WebPresentationController.h
@@ -10,6 +10,7 @@
 namespace blink {
 
 class WebPresentationSessionClient;
+class WebString;
 enum class WebPresentationSessionState;
 
 // The delegate Blink provides to WebPresentationClient in order to get updates.
@@ -30,6 +31,9 @@ public:
 
     // Called when the state of a session changes.
     virtual void didChangeSessionState(WebPresentationSessionClient*, WebPresentationSessionState) = 0;
+
+    // Called when a text message of a session is received.
+    virtual void didReceiveSessionTextMessage(WebPresentationSessionClient*, const WebString& message) = 0;
 };
 
 } // namespace blink