All the usages of this flag library have been replaced by ABSL_FLAG.
Bug: webrtc:10616
Change-Id: I7b50772654d68e80055d79f46ef5bc0633e75508
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/143482
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28401}