Unreviewed GTK build fix after r159889.

* UIProcess/gtk/WebPageProxyGtk.cpp:
(WebKit::WebPageProxy::viewWidget): Adjust the static cast of the PageClient reference to PageClientImpl.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@159890 268f45cc-cd09-0410-ab3c-d52691b4dbfc