Backport some tests in /chromeos/dbus/auth_policy 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/dbus/auth_policy.

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

This CL was uploaded by git cl split.

R=tnagel@chromium.org

Bug: 891670
Change-Id: I4b88ebcfc2c108c02a6ff8987d121f24bb7c5195
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786827
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Thiemo Nagel <tnagel@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694771}

chromeos/dbus/auth_policy/fake_auth_policy_client_unittest.cc

@@ -98,7 +98,7 @@ class FakeAuthPolicyClientTest : public ::testing::Test {
 
  private:
   ScopedStubInstallAttributes install_attributes_;
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
 
   DISALLOW_COPY_AND_ASSIGN(FakeAuthPolicyClientTest);
 };