[Presentation API] Adds close method to Presentation session.

Source/modules/presentation/PresentationController.h

Index: Source/modules/presentation/PresentationController.h
diff --git a/Source/modules/presentation/PresentationController.h b/Source/modules/presentation/PresentationController.h
index 89846e7a5d750685e7ef5b143c3def65532d2b1b..2ddec1fc96c80b8e419af554dadfb5292f4adcd5 100644
--- a/Source/modules/presentation/PresentationController.h
+++ b/Source/modules/presentation/PresentationController.h
@@ -54,6 +54,9 @@ public:
     // 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*);