Refactor creation of the full screen window into an (overridable) method.
https://bugs.webkit.org/show_bug.cgi?id=110369

Reviewed by Eric Carlson.

Add a new WKView method -(NSWindow)createFullScreenWindow which can be overridden
by subclasses to provide custom full screen windows to the WKFullScreenWindowController.

* UIProcess/API/mac/WKView.mm:
(-[WKView fullScreenWindowController]):
(-[WKView createFullScreenWindow]):
* UIProcess/API/mac/WKViewPrivate.h:
* UIProcess/mac/WKFullScreenWindowController.mm:
(-[WKFullScreenWindowController initWithWindow:]):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148901 268f45cc-cd09-0410-ab3c-d52691b4dbfc