[Sheriff] Disable flaky test PlatformThreadTest

Disable PlatformThreadWinTest.SetBackgroundThreadModeFailsInIdlePriorityProcess,
which is failing on Windows.

TBR=fdoray@chromium.org

Bug: 931720
Change-Id: I77fabc5c796b949f1616e0daf262695bf22ea14b
Reviewed-on: https://chromium-review.googlesource.com/c/1471385
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632027}
1 file changed