[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/1471385Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632027}

base/threading/platform_thread_win_unittest.cc

@@ -26,7 +26,9 @@ 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.
-TEST(PlatformThreadWinTest, SetBackgroundThreadModeFailsInIdlePriorityProcess) {
+// Fails on various windows bots: https://crbug.com/931720.
+TEST(PlatformThreadWinTest,
+     DISABLED_SetBackgroundThreadModeFailsInIdlePriorityProcess) {
   PlatformThreadHandle::Handle thread_handle =
       PlatformThread::CurrentHandle().platform_handle();