Set web contents font setting to default

Chromium has since version 50 overridden the global font settings with
the settings of every web contents created, so we need to first set the
global settings in the web contents, to still follow system defaults.

Task-number: QTBUG-62146
Change-Id: If10847c81beda08ecb7bfdf7556e39b1d2989754
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>