Backport some tests in /chrome/browser/chromeos/wilco_dtc_supportd 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/chromeos/wilco_dtc_supportd.

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

This CL was uploaded by git cl split.

Edit: Revert changes to wilco_dtc_supportd_manager_unittest.cc
      which now needs base::ThreadPool.

R=lamzin@google.com

Bug: 891670
Change-Id: I9e9400df3a840f82a3dbbf029449046914faf42d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786848
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Oleh Lamzin <lamzin@google.com>
Cr-Commit-Position: refs/heads/master@{#694576}

chrome/browser/chromeos/wilco_dtc_supportd/wilco_dtc_supportd_web_request_service_unittest.cc

@@ -150,7 +150,7 @@ class WilcoDtcSupportdWebRequestServiceTest : public testing::Test {
 
   std::unique_ptr<WilcoDtcSupportdWebRequestService> web_request_service_;
   network::TestURLLoaderFactory test_url_loader_factory_;
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
 };
 
 }  // namespace