Backport some tests in /media/gpu 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/gpu.

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

This CL was uploaded by git cl split.

R=kcwu@chromium.org

Bug: 891670
Change-Id: I1b3c3b43dc43e51b628e3e09767cc6f307b7794f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786957
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Kuang-che Wu <kcwu@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694571}

media/gpu/android/media_codec_video_decoder_unittest.cc

@@ -280,7 +280,7 @@ class MediaCodecVideoDecoderTest : public testing::TestWithParam<VideoCodec> {
 
  protected:
   const VideoCodec codec_;
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   base::android::ScopedJavaGlobalRef<jobject> java_surface_;
   scoped_refptr<DecoderBuffer> fake_decoder_buffer_;
   std::unique_ptr<MockDeviceInfo> device_info_;