Backport some tests in /media/capture/video 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/capture/video.

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

This CL was uploaded by git cl split.

R=emircan@chromium.org

Bug: 891670
Change-Id: I0fef3a26ba2a58d22bc0e19edbebe90f3d1589e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787181
Auto-Submit: Gabriel Charette <gab@chromium.org>
Commit-Queue: Emircan Uysaler <emircan@chromium.org>
Reviewed-by: Emircan Uysaler <emircan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695725}

media/capture/video/fake_video_capture_device_unittest.cc

@@ -200,7 +200,7 @@ class FakeVideoCaptureDeviceTestBase : public ::testing::Test {
 
   const VideoCaptureFormat& last_format() const { return last_format_; }
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   std::unique_ptr<VideoCaptureDeviceDescriptors> descriptors_;
   std::unique_ptr<base::RunLoop> run_loop_;
   std::unique_ptr<MockVideoCaptureDeviceClient> client_;

media/capture/video/video_capture_device_unittest.cc

@@ -250,7 +250,8 @@ class VideoCaptureDeviceTest
 #if defined(OS_MACOSX)
         // Video capture code on MacOSX must run on a CFRunLoop enabled thread
         // for interaction with AVFoundation.
-        task_environment_(base::test::TaskEnvironment::MainThreadType::UI),
+        task_environment_(
+            base::test::SingleThreadTaskEnvironment::MainThreadType::UI),
 #endif
         device_descriptors_(new VideoCaptureDeviceDescriptors()),
         main_thread_task_runner_(base::ThreadTaskRunnerHandle::Get()),
@@ -461,7 +462,7 @@ class VideoCaptureDeviceTest
 #if defined(OS_WIN)
   base::win::ScopedCOMInitializer initialize_com_;
 #endif
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   std::unique_ptr<VideoCaptureDeviceDescriptors> device_descriptors_;
   std::unique_ptr<base::RunLoop> run_loop_;
   scoped_refptr<base::TaskRunner> main_thread_task_runner_;