Backport some tests in /chromecast 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
/chromecast.

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

This CL was uploaded by git cl split.

R=servolk@chromium.org

Bug: 891670
Change-Id: Iaa04676f2f42a2be1353c0ac67cfeba6086143b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786985
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695744}

chromecast/base/bind_to_task_runner_unittest.cc

@@ -50,7 +50,7 @@ class BindToTaskRunnerTest : public ::testing::Test {
  public:
   ~BindToTaskRunnerTest() override { base::RunLoop().RunUntilIdle(); }
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   MockCallbacks callbacks_;
 };