Reactivate thread priority checks in cast 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@

Bug: 1097092
Change-Id: I6462e3aff2a0d1b0febc2eae7cd97bf580292768
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2254466Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781336}

chromecast/app/cast_test_launcher.cc

@@ -24,10 +24,8 @@ class CastTestLauncherDelegate : public content::TestLauncherDelegate {
 
   int RunTestSuite(int argc, char** argv) override {
     base::TestSuite test_suite(argc, argv);
-    // 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 .
     test_suite.DisableCheckForLeakedGlobals();
-    test_suite.DisableCheckForThreadPriorityAtTestEnd();
     return test_suite.Run();
   }