Backport some tests in /chromeos/services/network_config 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
/chromeos/services/network_config.

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

This CL was uploaded by git cl split.

R=khorimoto@chromium.org

Bug: 891670
Change-Id: I344d2168d15d303bab24507d348c85bdda2782ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786851
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693812}

chromeos/services/network_config/cros_network_config_unittest.cc

@@ -350,7 +350,7 @@ class CrosNetworkConfigTest : public testing::Test {
   std::string wifi1_path() { return wifi1_path_; }
 
  private:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   NetworkStateTestHelper helper_{false /* use_default_devices_and_services */};
   std::unique_ptr<NetworkProfileHandler> network_profile_handler_;
   std::unique_ptr<NetworkDeviceHandler> network_device_handler_;