Updated user agents (Firefox 52 ESR, Chromium 57)

Summary: New user agents, as the available ones are a bit old.

Test Plan:
Compiles, and the new user agents can be selected for example
in Konqueror. The generated user agent (at least on Linux-based
systems) matches the expected pattern.

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D5091