Add full SetThreadPriority support to Windows.

Also introducing the cross-platform GetThreadPriority() method to allow for
cross-platform tests for this code.

Enhanced PlatformThreadTests.

Discovering issue 468793 in the process where it appears that the POSIX priority
manipulation code doesn't actually work in practice...

BUG=456903, 468793, 338961, 468932

Review URL: https://codereview.chromium.org/1006933003

Cr-Commit-Position: refs/heads/master@{#323266}


CrOS-Libchrome-Original-Commit: f1921cb37b83bce16c07ca93975cded2b2863dd0
7 files changed
tree: 284af4f28183493d992930f06b43e8466edf08ef
  1. base/
  2. build/
  3. components/
  4. dbus/
  5. device/
  6. ipc/
  7. mojo/
  8. testing/
  9. third_party/
  10. ui/