[PresentationAPI] Added on-session-message handler.

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 65 matching lines @@
 
     // Called when closeSession() is called by the frame.
     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 text message.
+    ListenForSessionTextMessage()

[whywhat, 2015/04/23 16:52:27]

The Mojo document has something like this:

ListenForSessionTextMessage(PresentationSessionInfo sessionInfo) =>
(array[string] messages);

Using the array would handle the throttling problem (if the page is slow
receiving messages, we can send a batch of accumulated messages from the
browser/MR).
sessionInfo would probably be useful if we would only want to send messages to
the session that have onmessage event handlers set. However, I think we'd send
the messages to all the sessions so your version of passing the sessionInfo in
the callback is better.

So something like this then?

ListenForSessionTextMessage() => (PresentationSessionInfo sessionInfo,
array[string] messages);

[haibinlu, 2015/04/25 00:05:14]

Done. the callback parameter is array<SessionMessage>

+        => (PresentationSessionInfo sessionInfo, string message);

[whywhat, 2015/04/23 16:52:27]

Also consider making it a MessageRequest that would wrap string, arrays and
blobs? Could be left until the time we add support for those though.
Still with that in mind probably can omit Text from the method names.

[haibinlu, 2015/04/25 00:05:14]

Added SessionMessage, borrowed from send-message CL. Will merge

 };