[PresentationAPI] Added on-session-message handler.

content/common/presentation/presentation_service.mojom

Index: content/common/presentation/presentation_service.mojom
diff --git a/content/common/presentation/presentation_service.mojom b/content/common/presentation/presentation_service.mojom
index 36756e23a080698ec63f93627b099bba23d9750e..8010dc90347f252954e9c0f98aba6219b9f909a2 100644
--- a/content/common/presentation/presentation_service.mojom
+++ b/content/common/presentation/presentation_service.mojom
@@ -83,4 +83,8 @@ interface PresentationService {
     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

 };