Backport some tests in /components/favicon 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/favicon.

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

This CL was uploaded by git cl split.

R=mastiz@chromium.org

Bug: 891670
Change-Id: I9a8ef1f34217a8d8110454a28138963c99b66ca4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787060
Auto-Submit: Gabriel Charette <gab@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694652}

components/favicon/core/favicon_handler_unittest.cc

@@ -492,7 +492,8 @@ class FaviconHandlerTest : public testing::Test {
   const GURL kIconURL64x64 = GURL("http://www.google.com/favicon64x64");
 
   FaviconHandlerTest()
-      : task_environment_(base::test::TaskEnvironment::MainThreadType::UI) {
+      : task_environment_(
+            base::test::SingleThreadTaskEnvironment::MainThreadType::UI) {
     // Register various known icon URLs.
     delegate_.fake_image_downloader().Add(kIconURL10x10, IntVector{10});
     delegate_.fake_image_downloader().Add(kIconURL12x12, IntVector{12});
@@ -546,7 +547,7 @@ class FaviconHandlerTest : public testing::Test {
                                     candidates, manifest_url);
   }
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   std::unique_ptr<ui::test::ScopedSetSupportedScaleFactors>
       scoped_set_supported_scale_factors_;
   testing::NiceMock<MockFaviconServiceWithFake> favicon_service_;

components/favicon/core/favicon_service_impl_unittest.cc

@@ -17,7 +17,7 @@ namespace favicon {
 namespace {
 
 TEST(FaviconServiceImplTest, ShouldCacheUnableToDownloadFavicons) {
-  base::test::TaskEnvironment task_environment;
+  base::test::SingleThreadTaskEnvironment task_environment;
   base::ScopedTempDir history_dir;
   CHECK(history_dir.CreateUniqueTempDir());
   std::unique_ptr<history::HistoryService> history_service =