[base] Fix a crash in ScopedTaskEnvironmentTest.LifetimeObserver test.

Call ScopedTaskEnvironment::SetLifetimeObserver(nullptr) to ensure that
the next test after ScopedTaskEnvironmentTest.LifetimeObserver doesn't
crash.

R=gab@chromium.org
BUG=905812

Change-Id: Ifbdcfa55695fbd5641ffccb8d3e60a80d1afb3ca
Reviewed-on: https://chromium-review.googlesource.com/c/1338090
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608899}

base/test/scoped_task_environment_unittest.cc

@@ -346,6 +346,7 @@ TEST_F(ScopedTaskEnvironmentTest, LifetimeObserver) {
   EXPECT_CALL(lifetime_observer, OnScopedTaskEnvironmentDestroyed());
   task_environment.reset();
   testing::Mock::VerifyAndClearExpectations(&lifetime_observer);
+  ScopedTaskEnvironment::SetLifetimeObserver(nullptr);
 }
 
 INSTANTIATE_TEST_CASE_P(