Backport some tests in /components/unified_consent 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/unified_consent.

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

This CL was uploaded by git cl split.

R=tangltom@chromium.org

Bug: 891670
Change-Id: I34de3ff3201c5cac61dd42f55497f92de4d63017
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786823
Auto-Submit: Gabriel Charette <gab@chromium.org>
Commit-Queue: Thomas Tangl <tangltom@chromium.org>
Reviewed-by: Thomas Tangl <tangltom@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695110}

components/unified_consent/unified_consent_service_unittest.cc

@@ -89,7 +89,7 @@ class UnifiedConsentServiceTest : public testing::Test {
   }
 
  protected:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   sync_preferences::TestingPrefServiceSyncable pref_service_;
   signin::IdentityTestEnvironment identity_test_environment_;
   TestSyncService sync_service_;