Backport some tests in /components/network_time 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/network_time.

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

This CL was uploaded by git cl split.

R=zea@chromium.org

Bug: 891670
Change-Id: I444ad4c122f248c1a1116752cc253d55da465708
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786743
Auto-Submit: Gabriel Charette <gab@chromium.org>
Commit-Queue: Nicolas Zea <zea@chromium.org>
Reviewed-by: Nicolas Zea <zea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695722}

components/network_time/network_time_tracker_unittest.cc

@@ -48,7 +48,8 @@ class NetworkTimeTrackerTest : public ::testing::Test {
   ~NetworkTimeTrackerTest() override {}
 
   NetworkTimeTrackerTest()
-      : task_environment_(base::test::TaskEnvironment::MainThreadType::IO),
+      : task_environment_(
+            base::test::SingleThreadTaskEnvironment::MainThreadType::IO),
         field_trial_test_(new FieldTrialTest()),
         clock_(new base::SimpleTestClock),
         tick_clock_(new base::SimpleTestTickClock),
@@ -155,7 +156,7 @@ class NetworkTimeTrackerTest : public ::testing::Test {
   }
 
  protected:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   std::unique_ptr<FieldTrialTest> field_trial_test_;
   base::TimeDelta resolution_;
   base::TimeDelta latency_;