[Presentation API] Adds close method to Presentation session.

Source/modules/presentation/PresentationController.h

 // 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.
 
 #ifndef PresentationController_h
 #define PresentationController_h
 
 #include "core/frame/FrameDestructionObserver.h"
 #include "modules/presentation/Presentation.h"
 #include "platform/Supplementable.h"
@@ skipping 36 matching lines @@
     // Called when the first listener was added to or the last listener was removed from the
     // |availablechange| event.
     void updateAvailableChangeWatched(bool watched);
 
     // Called when the frame wants to start a new presentation.
     void startSession(const String& presentationUrl, const String& presentationId, WebPresentationSessionClientCallbacks*);
 
     // Called when the frame wants to join an existing presentation.
     void joinSession(const String& presentationUrl, const String& presentationId, WebPresentationSessionClientCallbacks*);
 
+    // Called when the frame wants to close an existing presentation.
+    void closeSession(const String& presentationId);

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

Shouldn't you pass the url too? I don't think we rely on just the ids to be
unique, only <url, id> pairs.
At least that's what i got from mfoltz@ and imcheng@ comments on
https://codereview.chromium.org/979413002/ We could discuss in more detail
tomorrow.

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

Done. But I do not think URL+ID is unique in a browser instance. URL is just
extra piece of information MR can use.

+
     // Connects the |Presentation| object with this controller.
     void setPresentation(Presentation*);
 
 private:
     PresentationController(LocalFrame&, WebPresentationClient*);
 
     // Implementation of FrameDestructionObserver.
     virtual void willDetachFrameHost() override;
 
     WebPresentationClient* m_client;
     PersistentWillBeMember<Presentation> m_presentation;
 };
 
 } // namespace blink
 
 #endif // PresentationController_h