Create FieldTrials class for setting the field trials from a string
in a program. We can later e.g add a builder class where one
can add key/value pairs.
This class is supposed to replace
webrtc::field_trial::InitFieldTrialsFromString.
No-Try: True
Bug: webrtc:10335
Change-Id: I17f45e401102fddda50ca7c4a04bea2f1cb87788
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/256973
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36376}
