Deduplicate PlatformThread's POSIX implementations of SetThreadPriority.

This deduplication becomes a particularly good win when implementing
GetThreadPriority() in https://codereview.chromium.org/1006933003/

BUG=456903,468793

Review URL: https://codereview.chromium.org/1015503002

Cr-Commit-Position: refs/heads/master@{#321403}


CrOS-Libchrome-Original-Commit: bb0d0b93645861e9e4dc66d31b674fdf2965ec4e
6 files changed
tree: d524a93ffac384b75005da5d928eabadca9c7a8b
  1. base/
  2. build/
  3. components/
  4. dbus/
  5. device/
  6. ipc/
  7. mojo/
  8. testing/
  9. third_party/
  10. ui/