Backport some tests in /media/remoting 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
/media/remoting.

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

This CL was uploaded by git cl split.

R=miu@chromium.org

Bug: 891670
Change-Id: I2093f1325393019e14ccb6e3b4f057401a0892c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787139
Auto-Submit: Gabriel Charette <gab@chromium.org>
Commit-Queue: Yuri Wiitala <miu@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695804}

media/remoting/courier_renderer_unittest.cc

@@ -389,7 +389,7 @@ class CourierRendererTest : public testing::Test {
     RunPendingTasks();
   }
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   std::unique_ptr<RendererController> controller_;
   std::unique_ptr<RendererClientImpl> render_client_;
   std::unique_ptr<FakeMediaResource> media_resource_;