Fix flaky SingleSampleMetricsFactoryImpl DCHECK_DEATH test.

Test was missing the FLAGS_gtest_death_test_style = "threadsafe"
declaration that's necessary for multi-threaded death tests. See
https://github.com/google/googletest/blob/master/googletest/docs/advanced.md#death-tests-and-threads

R=isherman

Fixed: 1009360
Change-Id: I795e055c9fdfda727853ce52c1278dd430cbfdef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2049023
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740119}

components/metrics/single_sample_metrics_factory_impl_unittest.cc

@@ -140,8 +140,11 @@ TEST_F(SingleSampleMetricsFactoryImplTest, DefaultSingleSampleMetricWithValue) {
                 base::HistogramBase::kUmaTargetedHistogramFlag));
 }
 
-// TODO(crbug.com/1009360). Flaky timeouts.
-TEST_F(SingleSampleMetricsFactoryImplTest, DISABLED_MultithreadedMetrics) {
+TEST_F(SingleSampleMetricsFactoryImplTest, MultithreadedMetrics) {
+  // Allow EXPECT_DCHECK_DEATH for multiple threads.
+  // https://github.com/google/googletest/blob/master/googletest/docs/advanced.md#death-tests-and-threads
+  testing::FLAGS_gtest_death_test_style = "threadsafe";
+
   base::HistogramTester tester;
   std::unique_ptr<base::SingleSampleMetric> metric =
       factory_->CreateCustomCountsMetric(kMetricName, kMin, kMax, kBucketCount);