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

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 99ab552c37f40c542d3588b9c14c0189abb8df32..407715168510a0ed375d803fbecd026c5a1754bf 100644
--- a/content/common/presentation/presentation_service.mojom
+++ b/content/common/presentation/presentation_service.mojom
@@ -70,6 +70,9 @@ interface PresentationService {
     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);
 };