[WK2] add cocoa API for private browsing setting

https://bugs.webkit.org/show_bug.cgi?id=127391

Reviewed by Sam Weinig.

* UIProcess/API/Cocoa/WKBrowsingContextGroup.h:
* UIProcess/API/Cocoa/WKBrowsingContextGroup.mm:
(-[WKBrowsingContextGroup privateBrowsingEnabled]):
(-[WKBrowsingContextGroup setPrivateBrowsingEnabled:]):
Adding some API for the private Browsing setting. Reaches through
to the WKPageGroup's preferences.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@162546 268f45cc-cd09-0410-ab3c-d52691b4dbfc