Backport some tests in /content/common/service_manager 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
/content/common/service_manager.

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

This CL was uploaded by git cl split.

R=rockot@google.com

Bug: 891670
Change-Id: I26e756b427a41a07ced80e5d9306788644d26ba0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786956
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694948}

content/common/service_manager/service_manager_connection_impl_unittest.cc

@@ -23,7 +23,7 @@ constexpr char kTestServiceName[] = "test service";
 }  // namespace
 
 TEST(ServiceManagerConnectionImplTest, ServiceLaunchThreading) {
-  base::test::TaskEnvironment task_environment;
+  base::test::SingleThreadTaskEnvironment task_environment;
   base::Thread io_thread("ServiceManagerConnectionImplTest IO Thread");
   io_thread.Start();
   service_manager::mojom::ServicePtr service;