Backport some tests in /chrome/browser/android/oom_intervention 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
/chrome/browser/android/oom_intervention.

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

This CL was uploaded by git cl split.

R=bashi@chromium.org

Bug: 891670
Change-Id: I3440ef4d31cea3e1ddeed8ce3fca8f871e4167e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786853
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694561}

chrome/browser/android/oom_intervention/near_oom_monitor_unittest.cc

@@ -82,7 +82,7 @@ class NearOomMonitorTest : public testing::Test {
 
  protected:
   scoped_refptr<base::TestMockTimeTaskRunner> task_runner_;
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   std::unique_ptr<MockNearOomMonitor> monitor_;
 };