Reactivate thread priority checks in ash shell test

Thread priority should not be modified after a test.
If that is happening, it needs to be fixed in the code.

This CL remove the code that disable the priority checks.

NOTE TO SHERIFF:
 * please paste failure logs when reverting, the goal is
   to remove flaky tests related to thread priority.
 * assigned flaky test to etienneb@

R=gab@chromium.org

Bug: 1097092
Change-Id: I0524298abd36ca6dcd3ba55bedd0e087f33ef97b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2262255
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782619}

ash/shell/content/test/ash_content_perf_test_launcher.cc

@@ -31,10 +31,8 @@ class AshContentTestSuite : public content::ContentTestSuiteBase {
  protected:
   // content::ContentTestSuiteBase:
   void Initialize() override {
-    // Browser tests are expected not to tear-down various globals and may
-    // complete with the thread priority being above NORMAL.
+    // Browser tests are expected not to tear-down various globals.
     base::TestSuite::DisableCheckForLeakedGlobals();
-    base::TestSuite::DisableCheckForThreadPriorityAtTestEnd();
     ContentTestSuiteBase::Initialize();
     ui_controls::InstallUIControlsAura(ash::test::CreateAshUIControls());
   }