[Presentation API] Adds close method to Presentation session.

Source/modules/presentation/PresentationController.cpp

 // 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.
 
 #include "config.h"
 #include "modules/presentation/PresentationController.h"
 
 #include "core/frame/LocalFrame.h"
 #include "modules/presentation/PresentationSession.h"
 #include "public/platform/modules/presentation/WebPresentationClient.h"
@@ skipping 86 matching lines @@
 
 void PresentationController::joinSession(const String& presentationUrl, const String& presentationId, WebPresentationSessionClientCallbacks* callbacks)
 {
     if (!m_client) {
         delete callbacks;
         return;
     }
     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;
 }
 
 void PresentationController::willDetachFrameHost()
 {
     if (m_client) {
         m_client->setController(nullptr);
         m_client = nullptr;
     }
 }
 
 } // namespace blink