Revert "[Sheriff] Disable flaky test PlatformThreadTest"

This reverts commit cd3f8bf2.

Reason for revert: I suspect that this has the same root cause
as https://crbug.com/931721 and should no longer fail with the
checks added in
https://chromium-review.googlesource.com/c/chromium/src/+/1790331.

Original change's description:
> [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}

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 931720
Change-Id: Icb313b8b6cc2bd09c929ebba969ede4ac134d9f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1804781
Auto-Submit: François Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696873}

base/threading/platform_thread_win_unittest.cc

@@ -24,9 +24,7 @@ namespace base {
 // behavior which we suspect is a Windows kernel bug. If this test starts
 // failing, the mitigation for https://crbug.com/901483 in
 // PlatformThread::SetCurrentThreadPriority() should be revisited.
-// Fails on various windows bots: https://crbug.com/931720.
-TEST(PlatformThreadWinTest,
-     DISABLED_SetBackgroundThreadModeFailsInIdlePriorityProcess) {
+TEST(PlatformThreadWinTest, SetBackgroundThreadModeFailsInIdlePriorityProcess) {
   PlatformThreadHandle::Handle thread_handle =
       PlatformThread::CurrentHandle().platform_handle();