Backport GTK3 patch from Ubuntu (See LP: #1736607)

Thanks for Mitsuya Shibata.

See https://bugs.launchpad.net/ubuntu/+source/mozc/+bug/1736607

Tested base minimum with:

budgie-desktop 10.9: fcitx5-mozc (x11) gnome: ibus-mozc (wayland) gnome flashback: ibus-mozc (x11) kde: ibus-mozc (wayland)