webview: don't set the OSD style class on the whole overlay
webview: don't set the OSD style class on the whole overlay

The progressbar is an OSD; setting it on the whole overlay means the
scrolled window and the view itself will also match it, which we don't
want. See also

http://git.gnome.org/browse/epiphany/commit/?id=2e710ab607b2ab2b18125574bf9fbcd44abd74b4

https://bugzilla.gnome.org/show_bug.cgi?id=691122

Distrotech/gnome-online-accounts,dnohales/gnome-online-accounts,dnohales/gnome-online-accounts,dnohales/gnome-online-accounts,felipeborges/gnome-online-accounts,felipeborges/gnome-online-accounts,halfline/gnome-online-accounts,Distrotech/gnome-online-accounts,halfline/gnome-online-accounts,ritzk/gnome-online-accounts,ritzk/gnome-online-accounts,halfline/gnome-online-accounts,felipeborges/gnome-online-accounts,ritzk/gnome-online-accounts,Distrotech/gnome-online-accounts