Backport some tests in /components/proxy_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
/components/proxy_config.

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

This CL was uploaded by git cl split.

R=stevenjb@chromium.org

Bug: 891670
Change-Id: Ia7042e262defa564a1dde6f1cf183dd8078979b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786845
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694568}

components/proxy_config/pref_proxy_config_tracker_impl_unittest.cc

@@ -104,7 +104,7 @@ class PrefProxyConfigTrackerImplTest : public testing::Test {
     proxy_config_service_.reset();
   }
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   std::unique_ptr<TestingPrefServiceSimple> pref_service_;
   TestProxyConfigService* delegate_service_; // weak
   std::unique_ptr<net::ProxyConfigService> proxy_config_service_;