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