Backport some tests in /media/cast 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/cast.

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: I6bd10bb9bfbc2d47f609640df14c2950db3541c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787210
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@{#695800}

media/cast/sender/h264_vt_encoder_unittest.cc

@@ -253,7 +253,7 @@ class H264VideoToolboxEncoderTest : public ::testing::Test {
   static FrameSenderConfig video_sender_config_;
 
   base::SimpleTestTickClock clock_;
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   scoped_refptr<CastEnvironment> cast_environment_;
   std::unique_ptr<VideoEncoder> encoder_;
   OperationalStatus operational_status_;

media/cast/test/end2end_unittest.cc

@@ -880,7 +880,7 @@ class End2EndTest : public ::testing::Test {
   std::vector<std::pair<base::TimeTicks, base::TimeTicks> > video_ticks_;
 
   // |transport_sender_| has a RepeatingTimer which needs a MessageLoop.
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
 };
 
 namespace {