[PresentationAPI] Implementing send() from WebPresentationClient to the PresentationService.

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 52 matching lines @@
     StartSession(string presentation_url, string? presentation_id)
         => (PresentationSessionInfo? sessionInfo, PresentationError? error);
 
     // Called when joinSession() is called by the frame. The result callback
     // works the same as for the method above. JoinSession will join a known
     // session (i.e. when the page navigates or the user opens another tab)
     // silently and without user action.
     JoinSession(string presentation_url, string? presentation_id)
         => (PresentationSessionInfo? sessionInfo, PresentationError? error);
 
+    // Called when postMessage(DOMString) is called by the frame.
+    PostMessage(string presentation_url, string presentation_id, string message);

[whywhat, 2015/03/26 00:24:14]

nit: fit into 80 lines

Mojo guidelines suggest we get a void result from the service to know when it's
safe to send the next message over the Mojo channel (e.g. if the service gets
stuck sending the message, the Mojo channel is not overloaded with messages
submitted via postMessage()). This also means we could potentially send an array
of messages to avoid calling the method many times if there's a queue:

PostMessage(string presentation_url, string presentation_id, array[string]
messages) => ();

implementation wise it means some queueing of messages mapped by url/id pair is
needed in the PresentationDispatcher

+
     // Called when closeSession() is called by the frame.
     CloseSession(string presentation_url, string presentation_id);
 };