webkitgtk: 2.20.2 -> 2.20.3

This addresses several issues with webkitgtk 2.20.2:

- CVE-2018-4190, information leak (credentials)
- CVE-2018-4199, arbitrary code execution
- CVE-2018-4218, arbitrary code execution
- CVE-2018-4222, arbitrary code execution
- CVE-2018-4232, cookies might be overriden
- CVE-2018-4233, arbitrary code execution
- CVE-2018-11646, application crash
- CVE-2018-12293, heap buffer overflow