Backport some tests in /components/offline_pages 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/offline_pages.

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

This CL was uploaded by git cl split.

R=chili@chromium.org

Bug: 891670
Change-Id: Ie41dd4e6a7e49c643df9a45421b6f7fef1b57c6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786822
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Cathy Li <chili@chromium.org>
Commit-Queue: Cathy Li <chili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693832}

components/offline_pages/core/prefetch/prefetch_gcm_app_handler_unittest.cc

@@ -62,7 +62,7 @@ class PrefetchGCMAppHandlerTest : public testing::Test {
   TestTokenFactory* token_factory() { return token_factory_; }
 
  private:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   std::unique_ptr<PrefetchServiceTestTaco> prefetch_service_taco_;
 
   // Owned by the taco.

components/offline_pages/core/prefetch/suggested_articles_observer_unittest.cc

@@ -69,7 +69,7 @@ class OfflinePageSuggestedArticlesObserverTest : public testing::Test {
       Category::FromKnownCategory(ntp_snippets::KnownCategories::ARTICLES);
 
  private:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   std::unique_ptr<PrefetchServiceTestTaco> prefetch_service_test_taco_;
 
   // Owned by the PrefetchServiceTestTaco.