Reactivate thread priority checks in interactive_ui_tests

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@

Bug: 1097092
Change-Id: Id39eac9370d7f554ea16562f422818a44a539adc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2254579
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780383}

chrome/test/base/interactive_ui_tests_main.cc

@@ -41,10 +41,8 @@ class InteractiveUITestSuite : public ChromeTestSuite {
  protected:
   // ChromeTestSuite overrides:
   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();
 
     ChromeTestSuite::Initialize();