Backport some tests in /components/omnibox 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/omnibox.

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

This CL was uploaded by git cl split.

Edit: Revert changes to HQPPerfTestOnePopularURL which needs
      base::ThreadPool now.

R=jdonnelly@chromium.org

Bug: 891670
Change-Id: I625d146aca749921b09a9af2ec8ed68d355bcf10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786756
Commit-Queue: Justin Donnelly <jdonnelly@chromium.org>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694750}

components/omnibox/browser/omnibox_controller_unittest.cc

@@ -33,7 +33,7 @@ class OmniboxControllerTest : public testing::Test {
   void SetUp() override;
   void TearDown() override;
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   std::unique_ptr<TestOmniboxClient> omnibox_client_;
   std::unique_ptr<OmniboxController> omnibox_controller_;
 

components/omnibox/browser/zero_suggest_provider_unittest.cc

@@ -152,7 +152,7 @@ class ZeroSuggestProviderTest : public testing::Test,
   void CreateMostVisitedFieldTrial();
   void SetZeroSuggestVariantForAllContexts(const std::string& variant);
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   std::unique_ptr<base::test::ScopedFeatureList> scoped_feature_list_;
 
   std::unique_ptr<FakeAutocompleteProviderClient> client_;