Backport some tests in /components/ntp_tiles 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/ntp_tiles.

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

This CL was uploaded by git cl split.

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

R=dbeam@chromium.org

Bug: 891670
Change-Id: I3ebb472b6163cebe6a3d7143a54ee72cf4fa8517
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787197
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694582}

components/ntp_tiles/most_visited_sites_unittest.cc

@@ -559,7 +559,7 @@ class MostVisitedSitesTest : public ::testing::TestWithParam<bool> {
       suggestions_service_callbacks_;
   TopSitesCallbackList top_sites_callbacks_;
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   data_decoder::TestingJsonParser::ScopedFactoryOverride factory_override_;
   sync_preferences::TestingPrefServiceSyncable pref_service_;
   PopularSitesFactoryForTest popular_sites_factory_;

components/ntp_tiles/popular_sites_impl_unittest.cc

@@ -212,8 +212,8 @@ class PopularSitesTest : public ::testing::Test {
   const TestPopularSite kYouTube;
   const TestPopularSite kChromium;
 
-  base::test::TaskEnvironment task_environment_{
-      base::test::TaskEnvironment::MainThreadType::UI};
+  base::test::SingleThreadTaskEnvironment task_environment_{
+      base::test::SingleThreadTaskEnvironment::MainThreadType::UI};
   data_decoder::TestingJsonParser::ScopedFactoryOverride factory_override_;
   std::unique_ptr<sync_preferences::TestingPrefServiceSyncable> prefs_;
   network::TestURLLoaderFactory test_url_loader_factory_;