Backport some tests in /components/metrics to SingleThreadTaskEnvironment

These tests were previously migrated from single-threaded MessageLoop to
a multi-threaded TaskEnvironment (then named ScopedTaskEnvironment) as
part of crbug.com/891670.

//base OWNERS decided in retrospect that it was better to keep a
single-threaded option for TaskEnvironment and introduced
SingleThreadTaskEnvironment. This CL retrofits that decision for
/components/metrics.

This CL is a no-op if it passes CQ.

This CL was uploaded by git cl split.

R=nikunjb@chromium.org

Bug: 891670
Change-Id: Ia47c792059528981fcbe3b0ca40baadfece43154
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786976
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Nik Bhagat <nikunjb@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696024}

components/metrics/single_sample_metrics_factory_impl_unittest.cc

@@ -75,7 +75,7 @@ class SingleSampleMetricsFactoryImplTest : public testing::Test {
                                                  kBucketCount);
   }
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   SingleSampleMetricsFactoryImpl* factory_;
   base::Thread thread_;
   size_t provider_count_ = 0;