Backport some tests in...

Backport some tests in /content/browser/renderer_host/media/video_capture_buffer_pool_unittest.cc 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
/content/browser/renderer_host/media/video_capture_buffer_pool_unittest.cc.

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

This CL was uploaded by git cl split.

R=mcasas@chromium.org

Bug: 891670
Change-Id: I0b56ac7f2bf659f28ebdbf8a04e425301033b9ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787114
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696025}

content/browser/renderer_host/media/video_capture_buffer_pool_unittest.cc

@@ -93,7 +93,7 @@ class VideoCaptureBufferPoolTest
         new Buffer(pool_, std::move(buffer_handle), buffer_id));
   }
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   int expected_dropped_id_;
   scoped_refptr<media::VideoCaptureBufferPool> pool_;