Reland "Fix flaky SingleSampleMetricsFactoryImpl DCHECK_DEATH test."

This reverts commit d709d76f.

Reason for revert: The culprit cannot cause that failure. Relanding...

Original change's description:
> Revert "Fix flaky SingleSampleMetricsFactoryImpl DCHECK_DEATH test."
> 
> This reverts commit 5381b40a.
> 
> Reason for revert: this CL probably caused GmockCallbackSupportTest.RunOnceClosureValueMultipleCall failures. 
> The first failed build https://ci.chromium.org/p/chrome/builders/ci/chromeos-eve-chrome/3283
> 
> Test 'GmockCallbackSupportTest.RunOnceClosureValueMultipleCall' completed with the following status(es): 'FAILURE','FAILURE'
> 
> Test 'GmockCallbackSupportTest.RunOnceClosureValueMultipleCall' had the following logs when run:
> 
> ================================================================================
> 
> [ RUN      ] GmockCallbackSupportTest.RunOnceClosureValueMultipleCall
> ../../base/test/gmock_callback_support_unittest.cc:174: Failure
> Death test: check.Call()
>     Result: died but not with expected error.
>   Expected: contains regular expression "copyable_cb->data"
> Actual msg:
> [  DEATH   ]
> Stack trace:
> #0 0x57c25d527cd4 base::test::GmockCallbackSupportTest_RunOnceClosureValueMultipleCall_Test::TestBody()
> 
> [  FAILED  ] GmockCallbackSupportTest.RunOnceClosureValueMultipleCall (26 ms)
> [ RUN      ] GmockCallbackSupportTest.RunOnceClosureValueMultipleCall
> ../../base/test/gmock_callback_support_unittest.cc:174: Failure
> Death test: check.Call()
>     Result: died but not with expected error.
>   Expected: contains regular expression "copyable_cb->data"
> Actual msg:
> [  DEATH   ]
> Stack trace:
> #0 0x58a33c3f9cd4 base::test::GmockCallbackSupportTest_RunOnceClosureValueMultipleCall_Test::TestBody()
> 
> [  FAILED  ] GmockCallbackSupportTest.RunOnceClosureValueMultipleCall (29 ms)
> 
> 
> Original change's description:
> > 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}
> 
> TBR=dalecurtis@chromium.org,isherman@chromium.org
> 
> Change-Id: I06415066abd98925dc7dcaac961e666ca68d6864
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2050365
> Reviewed-by: Maxim Kolosovskiy <kolos@chromium.org>
> Commit-Queue: Maxim Kolosovskiy <kolos@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#740270}

TBR=dalecurtis@chromium.org,isherman@chromium.org,kolos@chromium.org

Change-Id: I231b0c108d09f35a59216d08b401a28fbf2b4d31
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2050271Reviewed-by: Maxim Kolosovskiy <kolos@chromium.org>
Commit-Queue: Maxim Kolosovskiy <kolos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740294}

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);