Tidy up fOwner in SkMutex_pthread.h.

  - Initialize fOwner to kNoOwner in SkMutex::SkMutex.
  - Note that we're stretching the API with kNoOwner.
  - Use pthread_equal() instead of ==.

BUG=387993
R=halcanary@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/357473002