<https://webkit.org/b/119957> Page::contextMenuController() should return a reference.

Reviewed by Antti Koivisto.

Page::m_contextMenuController is never null.

* page/Page.h:
(WebCore::Page::contextMenuController):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@154239 268f45cc-cd09-0410-ab3c-d52691b4dbfc