[WK2] [GTK] Destructor not invoked in EditorClientFrameDestructionObserver

[WK2] [GTK] Destructor not invoked in EditorClientFrameDestructionObserver
https://bugs.webkit.org/show_bug.cgi?id=79466

Reviewed by Philippe Normand.

Explicitly cast the observer before deleting it instead of just
deleting a generic pointer. We must to that in order to get the
destructor of the object properly called.

* WebProcess/WebCoreSupport/gtk/WebEditorClientGtk.cpp:
(WebKit::EditorClientFrameDestructionObserver::destroyOnClosureFinalization):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108760 268f45cc-cd09-0410-ab3c-d52691b4dbfc