www/webkit2-gtk3: update to 2.30.5

Changelog:
https://webkitgtk.org/2021/02/11/webkitgtk2.30.5-released.html

• Bring back the WebKitPluginProcess intallation that was removed by mistake.
• Fix RunLoop objects leaked in worker threads.
• Fix aarch64 llint build with JIT disabled.
• Use Internet Explorer quirk for Google Docs.