Changes imported from Abseil "staging" branch:

  - 1a906f90b303c6fbac292f4883186a27e7c6476c Increase the mutex_test timeout (by removing the timeout ... by Derek Mauro <dmauro@google.com>
  - cfc1d3e56727e5b3a41b7d9dfc513f692ac0edd6 Require iOS 9.0 for ABSL_HAVE_THREAD_LOCAL. by Matt Armstrong <marmstrong@google.com>
  - 22dc24504ac6fbd2a22f5d45a4bd00fe14d500c3 Fix indentation by Jorg Brown <jorg@google.com>

GitOrigin-RevId: 1a906f90b303c6fbac292f4883186a27e7c6476c
Change-Id: Id6075001460bd6b9ce973897544269fa4103fee8