This replaces field_trial:: -based functions from system_wrappers. Field trials are still used as fallback, but injectable trials are now possible. Bug: webrtc:11926 Change-Id: I70f28c4fbabf6d9e55052342000e38612b46682c Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/174261 Reviewed-by: Per Kjellander <perkj@webrtc.org> Reviewed-by: Kári Helgason <kthelgason@webrtc.org> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org> Reviewed-by: Stefan Holmer <stefan@webrtc.org> Reviewed-by: Sebastian Jansson <srte@webrtc.org> Commit-Queue: Erik Språng <sprang@webrtc.org> Cr-Commit-Position: refs/heads/master@{#32129}