Backport some tests in /components/component_updater 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/component_updater.

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

This CL was uploaded by git cl split.

R=waffles@chromium.org

Bug: 891670
Change-Id: I4f3806776673850e73bd3d6966d1ae50b0b5e43d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786752
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Commit-Queue: Joshua Pawlicki <waffles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693676}

components/component_updater/timer_unittest.cc

@@ -19,11 +19,12 @@ namespace component_updater {
 class ComponentUpdaterTimerTest : public testing::Test {
  public:
   ComponentUpdaterTimerTest()
-      : task_environment_(base::test::TaskEnvironment::MainThreadType::UI) {}
+      : task_environment_(
+            base::test::SingleThreadTaskEnvironment::MainThreadType::UI) {}
   ~ComponentUpdaterTimerTest() override {}
 
  private:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
 };
 
 TEST_F(ComponentUpdaterTimerTest, Start) {