XFAIL tst_QWebEnginePage::backActionUpdate

The test relies on events being propagated from the QWebEngineView to
its children. We should probably send those events directly to the
WebContentsAdapter to fix this kind of use case.

Change-Id: Ib5cc199fe8fc27c21603888b49334d1c1f71e575
Reviewed-by: Michael Bruning <michael.bruning@digia.com>