www/chromium: some improvements for custom builds:

• enable link-time optimization (only for 13.0+ for now) [1]
• make Kerberos backend configurable (default unchanged) [2]

PR: 253628 [1]
PR: 254086 [2]
Submitted by: iron.udjin@gmail.com [1]
Submitted by: david@dcrosstech.com [2]