Update webkit-gtk3 to 2.26.3.

• Fix issues while trying to play a video on NextCloud.
• Make sure the GL video sink uses a valid WebKit shared GL context.
• Fix vertical alignment of text containing arabic diacritics.
• Fix build with icu 65.1.
• Fix page loading errors with websites using HSTS.
• Fix web process crash when displaying a KaTeX formula.
• Fix several crashes and rendering issues.

This release also fixes 3 CVE's.

MFH: 2020Q1
Security: dc8cff4c-4063-11ea-8a94-3497f6939fdd