Implements ListenForSessionMessages in PresentationServiceImpl; uses Swap to avoid copying large da…

content/common/presentation/presentation_service.mojom

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module presentation;
 
 struct PresentationSessionInfo {
     string url;
     string id;
 };
@@ skipping 80 matching lines @@
     CloseSession(string presentation_url, string presentation_id);
 
     // Called when the frame is ready to process the next state change. Returns
     // the last session state if it’s changed since the last time the callback
     // was called. Might cause the event fired with the initial state change.
     ListenForSessionStateChange()
         => (PresentationSessionInfo sessionInfo,
             PresentationSessionState newState);
 
     // Called when the frame is ready to process the next batch of messages.
+    // When the callback carries null messages, there is an error
+    // at the presentation service side.
     ListenForSessionMessages()
-        => (array<SessionMessage> messages);
+        => (array<SessionMessage>? messages);
 };