Backport some tests in /chrome/browser/android/contextualsearch 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
/chrome/browser/android/contextualsearch.

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

This CL was uploaded by git cl split.

R=donnd@chromium.org

Bug: 891670
Change-Id: I4eafad717c740b5168d1f61dd7ad65fdafd756fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786755
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Donn Denman <donnd@chromium.org>
Commit-Queue: Donn Denman <donnd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694315}

chrome/browser/android/contextualsearch/contextual_search_delegate_unittest.cc

@@ -288,8 +288,8 @@ class ContextualSearchDelegateTest : public testing::Test {
   std::string search_url_preload_;
   int coca_card_tag_;
 
-  base::test::TaskEnvironment task_environment_{
-      base::test::TaskEnvironment::MainThreadType::IO};
+  base::test::SingleThreadTaskEnvironment task_environment_{
+      base::test::SingleThreadTaskEnvironment::MainThreadType::IO};
   std::unique_ptr<TemplateURLService> template_url_service_;
   scoped_refptr<network::SharedURLLoaderFactory>
       test_shared_url_loader_factory_;