Fix flaky PromoServiceTests

Move the GoogleURLTracker shutdown from the test destructor
to TearDown(). This ensures it gets shutdown before
the checks in ~TestBrowserThreadBundle() run.

Bug: 937041
Change-Id: I6f43cc369f2e482de02771173d1bf2ae7237f70d
Reviewed-on: https://chromium-review.googlesource.com/c/1497220Reviewed-by: Kristi Park <kristipark@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636969}

chrome/browser/search/promos/promo_service_unittest.cc

@@ -57,10 +57,6 @@ class PromoServiceTest : public testing::Test {
             GoogleURLTracker::ALWAYS_DOT_COM_MODE,
             network::TestNetworkConnectionTracker::GetInstance()) {}
 
-  ~PromoServiceTest() override {
-    static_cast<KeyedService&>(google_url_tracker_).Shutdown();
-  }
-
   void SetUp() override {
     testing::Test::SetUp();
 
@@ -68,6 +64,10 @@ class PromoServiceTest : public testing::Test {
                                               &google_url_tracker_);
   }
 
+  void TearDown() override {
+    static_cast<KeyedService&>(google_url_tracker_).Shutdown();
+  }
+
   void SetUpResponseWithData(const GURL& load_url,
                              const std::string& response) {
     test_url_loader_factory_.SetInterceptor(base::BindLambdaForTesting(
@@ -100,8 +100,7 @@ class PromoServiceTest : public testing::Test {
   std::unique_ptr<PromoService> service_;
 };
 
-// Flaky on linux-chromeos-rel and win. See https://crbug.com/937041.
-TEST_F(PromoServiceTest, DISABLED_PromoDataNetworkError) {
+TEST_F(PromoServiceTest, PromoDataNetworkError) {
   SetUpResponseWithNetworkError(service()->GetLoadURLForTesting());
 
   ASSERT_EQ(service()->promo_data(), base::nullopt);