Backport some tests in...

Backport some tests in /components/web_resource/resource_request_allowed_notifier_unittest.cc 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/web_resource/resource_request_allowed_notifier_unittest.cc.

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

This CL was uploaded by git cl split.

R=asvitkine@chromium.org

Bug: 891670
Change-Id: I13509fa01b3fadb6cffa203cba15975ac553e8db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786777
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693837}

components/web_resource/resource_request_allowed_notifier_unittest.cc

@@ -119,8 +119,8 @@ class ResourceRequestAllowedNotifierTest
   }
 
  private:
-  base::test::TaskEnvironment task_environment_{
-      base::test::TaskEnvironment::MainThreadType::UI};
+  base::test::SingleThreadTaskEnvironment task_environment_{
+      base::test::SingleThreadTaskEnvironment::MainThreadType::UI};
   TestRequestAllowedNotifier resource_request_allowed_notifier_;
   TestingPrefServiceSimple prefs_;
   TestEulaAcceptedNotifier* eula_notifier_;  // Weak, owned by RRAN.