Fairshare mutexes performed really badly during a Catalina
performance test. This change switches them to use
the _PTHREAD_MUTEX_POLICY_FIRSTFIT policy instead.
Bug: webrtc:11567, webrtc:11648
Change-Id: I2b8fbe3183beefc26f8d4ff3d63dc6958174605f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/176504
Reviewed-by: Tommi <tommi@webrtc.org>
Reviewed-by: Oskar Sundbom <ossu@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31456}