www/webkit2-gtk3: Update to 2.28.4

Changes: https://webkitgtk.org/2020/04/24/webkitgtk2.28.2-released.html
Changes: https://webkitgtk.org/2020/07/09/webkitgtk2.28.3-released.html
Changes: https://webkitgtk.org/2020/07/28/webkitgtk2.28.4-released.html
PR: 246996
Submitted by: Marco Beishuizen <mbeis@xs4all.nl> (update to 2.28.2, 2.28.3)
Approved by: gnome (maintainer timeout, >1 month)