Prevent ScopedTempDir cleanup issues in FaviconServiceImplTest

Also switch from SingleThreadedTaskEnvironment to TaskEnvironment to
prepare for activation of HistoryServiceUsesTaskScheduler activation.

The feature will soon be flipped to ENABLED_BY_DEFAULT. This will make
HistoryServiceBackend's task runner dispatch to the ThreadPool instead
of a dedicated base::Thread. The tests need to be prepared to stay
functional when that happens.

Bug: 546640
Change-Id: I6c9c414794dc86c96721bdd23ccb31765e6a31c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1976290
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Auto-Submit: Oliver Li <olivierli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726417}

components/favicon/core/favicon_service_impl_unittest.cc

@@ -17,8 +17,8 @@ namespace favicon {
 namespace {
 
 TEST(FaviconServiceImplTest, ShouldCacheUnableToDownloadFavicons) {
-  base::test::SingleThreadTaskEnvironment task_environment;
   base::ScopedTempDir history_dir;
+  base::test::TaskEnvironment task_environment;
   CHECK(history_dir.CreateUniqueTempDir());
   std::unique_ptr<history::HistoryService> history_service =
       history::CreateHistoryService(history_dir.GetPath(), /*create_db=*/false);