This cl/ 1) move WebRtcKeyValueConfig from api/transport to api/ directory. 2) add a test/ScopedKeyValueConfig (compare ScopedFieldTrials). 3) removes usage of webrtc::field_trial:: from the pc/ directory. 4) removes a few unused includes of system_wrappers/field_trial.h. Bug: webrtc:10335 Change-Id: If29c07900dbe791050b0a5ad05332bedfad035f2 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/253903 Reviewed-by: Harald Alvestrand <hta@webrtc.org> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org> Commit-Queue: Jonas Oreland <jonaso@webrtc.org> Cr-Commit-Position: refs/heads/main@{#36160}