Revert "[Sheriff] Disable flaky test PlatformThreadTest"

This reverts commit b9310a10.

Reason for revert: The tests should no longer fail with
https://chromium-review.googlesource.com/c/chromium/src/+/1804786
ensuring that tests don't change the thread priority.

Original change's description:
> [Sheriff] Disable flaky test PlatformThreadTest
>
> Disable PlatformThreadTest.SetCurrentThreadPriorityWithThreadModeBackground
> and PlatformThreadTest.SetCurrentThreadPriorityWithThreadModeBackgroundIdleProcess
> (these tests are Windows-only).
>
> Bug: 931706
> TBR=fdoray@chromium.org
>
> Change-Id: I996a7faff6afc4ac161ec863a4a320e79f2a18c9
> Reviewed-on: https://chromium-review.googlesource.com/c/1471016
> Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
> Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#631860}

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

Bug: 931706
Change-Id: I6622ce09d03aa9e6ce6f776b245cfe12ea954b1b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1804787
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@{#696869}

base/threading/platform_thread_unittest.cc

@@ -289,9 +289,7 @@ TEST(PlatformThreadTest, MAYBE_SetCurrentThreadPriority) {
 #if defined(OS_WIN)
 // Test changing a created thread's priority, with the
 // kWindowsThreadModeBackground feature enabled.
-// Flaky: https://crbug.com/931706
-TEST(PlatformThreadTest,
-     DISABLED_SetCurrentThreadPriorityWithThreadModeBackground) {
+TEST(PlatformThreadTest, SetCurrentThreadPriorityWithThreadModeBackground) {
   test::ScopedFeatureList scoped_feature_list;
   scoped_feature_list.InitAndEnableFeature(
       features::kWindowsThreadModeBackground);
@@ -301,9 +299,8 @@ TEST(PlatformThreadTest,
 // Test changing a created thread's priority, with the
 // kWindowsThreadModeBackground feature enabled, in an IDLE_PRIORITY_CLASS
 // process (regression test for https://crbug.com/901483).
-// Flaky: https://crbug.com/931706
 TEST(PlatformThreadTest,
-     DISABLED_SetCurrentThreadPriorityWithThreadModeBackgroundIdleProcess) {
+     SetCurrentThreadPriorityWithThreadModeBackgroundIdleProcess) {
   ::SetPriorityClass(Process::Current().Handle(), IDLE_PRIORITY_CLASS);
 
   test::ScopedFeatureList scoped_feature_list;