Intent to Deprecate and Freeze: The User-Agent string

[fnando]

Looks like browsers are moving away from user agent strings in a sense; I'm adding this ticket so we're aware, in case anything breaks in the future.

https://groups.google.com/a/chromium.org/forum/m/#!msg/blink-dev/-2JIRNMWJ7s/yHe4tQNLCgAJ