[Presentation API] Adds close method to Presentation session.

Source/modules/presentation/PresentationController.cpp

Index: Source/modules/presentation/PresentationController.cpp
diff --git a/Source/modules/presentation/PresentationController.cpp b/Source/modules/presentation/PresentationController.cpp
index b5ccfa76bb74b0225a7e1cdb372ea23179f5a159..ca5958707049389895c9c22aacf1b213c4e72bcd 100644
--- a/Source/modules/presentation/PresentationController.cpp
+++ b/Source/modules/presentation/PresentationController.cpp
@@ -104,6 +104,14 @@ void PresentationController::joinSession(const String& presentationUrl, const St
     m_client->joinSession(presentationUrl, presentationId, callbacks);
 }
 
+void PresentationController::closeSession(const String& presentationId)
+{
+    if (!m_client) {

[whywhat, 2015/03/17 23:35:19]

nit: remove the curly braces to be consistent with other one line if's in the
file

[haibinlu, 2015/03/18 00:58:22]

Done.

+        return;
+    }
+    m_client->closeSession(presentationId);

[whywhat, 2015/03/17 23:35:19]

high level question: do we want to get any result back, btw?

[haibinlu, 2015/03/18 00:58:22]

The result of session.close is onstatechange event. Two approaches:
1. Have PresentationController.closeSession return a promise. Then fire
onstatechagne when promise resolves to true.
2. Have MR send onstatechange event if session is stopped.
PresentationController.closeSession return void. 

From developer point of view, the two approaches have no difference. I prefer
the 2nd approach because MR has to send onstatechange anyway in case the session
was stopped by other senders etc. For example, 

1. web app A in browser 1 invokes session.close; 
2. web app B in browser 2 invokes session.close;
3. right after browser 1 sends out stop command, the session stopped message
arrives as a broadcast message. browser 1 knows the session was stopped, but can
not relate the message to its previous command because it is a broadcast
message.
4. browser 1 updates web app A via onstatechange. The promise returned is still
unresolved.

[whywhat, 2015/03/18 12:51:19]

Ok, I think this makes sense and aligns with the spec.

+}
+
 void PresentationController::setPresentation(Presentation* presentation)
 {
     m_presentation = presentation;