Backport some tests in /base/fuchsia 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
/base/fuchsia.

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

This CL was uploaded by git cl split.

TBR=jamesr@chromium.org

Bug: 891670
Change-Id: I4a96de0332e48cde0453e04ec7552e2708a4f8f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787068Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694586}

base/fuchsia/service_directory_test_base.h

@@ -31,9 +31,8 @@ class ServiceDirectoryTestBase : public testing::Test {
  protected:
   const RunLoop::ScopedRunTimeoutForTest run_timeout_;
 
-  base::test::TaskEnvironment task_environment_{
-      base::test::TaskEnvironment::ThreadingMode::MAIN_THREAD_ONLY,
-      base::test::TaskEnvironment::MainThreadType::IO};
+  base::test::SingleThreadTaskEnvironment task_environment_{
+      base::test::SingleThreadTaskEnvironment::MainThreadType::IO};
 
   std::unique_ptr<sys::OutgoingDirectory> outgoing_directory_;
   TestInterfaceImpl test_service_;

base/fuchsia/service_provider_impl_unittest.cc

@@ -55,9 +55,8 @@ class ServiceProviderImplTest : public testing::Test {
   }
 
  protected:
-  base::test::TaskEnvironment task_environment_{
-      base::test::TaskEnvironment::ThreadingMode::MAIN_THREAD_ONLY,
-      base::test::TaskEnvironment::MainThreadType::IO};
+  base::test::SingleThreadTaskEnvironment task_environment_{
+      base::test::SingleThreadTaskEnvironment::MainThreadType::IO};
   TestInterfaceImpl test_service_;
 
   sys::OutgoingDirectory service_directory_;