[Sheriffing] Disable PostTaskTestWithExecutor.RegisterExecutorTwice on CrOS

TBR=gab@chromium.org

Bug: 879984
Change-Id: I994de9e8f5f9ac51fde51e7376b399d271f53181
Reviewed-on: https://chromium-review.googlesource.com/1201849Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588355}

base/task/post_task_unittest.cc

@@ -187,7 +187,13 @@ TEST_F(PostTaskTestWithExecutor, PostTaskToTaskExecutor) {
   }
 }
 
-TEST_F(PostTaskTestWithExecutor, RegisterExecutorTwice) {
+// Flaky on CrOS (https://crbug.com/879984).
+#if defined(OS_CHROMEOS)
+#define MAYBE_RegisterExecutorTwice DISABLED_RegisterExecutorTwice
+#else
+#define MAYBE_RegisterExecutorTwice RegisterExecutorTwice
+#endif
+TEST_F(PostTaskTestWithExecutor, MAYBE_RegisterExecutorTwice) {
   EXPECT_DCHECK_DEATH(
       RegisterTaskExecutor(TestTaskTraitsExtension::kExtensionId, &executor_));
 }