Backport some tests in /components/content_settings 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/content_settings.

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

This CL was uploaded by git cl split.

R=msramek@chromium.org

Bug: 891670
Change-Id: Icbf7ec34d5cec0751a115c88cddb1e57ad7afa64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786746
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Martin Šrámek <msramek@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694738}

components/content_settings/core/browser/cookie_settings_unittest.cc

@@ -95,7 +95,7 @@ class CookieSettingsTest : public testing::Test {
 
   // There must be a valid ThreadTaskRunnerHandle in HostContentSettingsMap's
   // scope.
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
 
   sync_preferences::TestingPrefServiceSyncable prefs_;
   scoped_refptr<HostContentSettingsMap> settings_map_;