navigator.userAgent deprecation

[fippo]

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

[fippo]

git grep browserDetails src/ isn't heavily used internally.

getUserAgent() returns a promise and is not sync which is a problem. We'd only use the major version anyway.

I'm not sure if there is much action here, adapter can continue to return the values from the UA but it needs to be documented that this isn't to be trusted. If browsers add a reliable way to detect the major version that would actually be great

[fippo]

Done in 9.0.0