fix QWebSettings::setUserStyleSheetUrl() not working with windows paths

fix QWebSettings::setUserStyleSheetUrl() not working with windows paths

QWebSettings::setUserStyleSheetUrl() does not work with windows paths
that contain drive letters.

KURL::path() alone does not handle removing the leading slash from a
windows file path. Using QUrl::toLocalFile() will turn
​file:///C:/path into C:/path appropriately

This fixes https://bugs.webkit.org/show_bug.cgi?id=34884.

Change-Id: I37a1482b67447be6ea6a10966cf59c9159db83cd
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>