Reviewed by Alexey Proskuryakov.

        The default backspace event handler should mark the event as handled if navigation succeeds.
        https://bugs.webkit.org/show_bug.cgi?id=58379

        We should mark the event as handled only if we were able to successfully navigate backwards or forwards.
        These navigations can fail if there is now back/forward history. This can occur in cases like ChromeFrame
        where history is managed by an external browser like IE.

        No new tests added as this scenario can occur when the history view is split across two browsers (WebKit and IE)
        It is non trivial to simulate this environment.

        * page/EventHandler.cpp:
        (WebCore::EventHandler::defaultBackspaceEventHandler):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@83688 268f45cc-cd09-0410-ab3c-d52691b4dbfc