Use ScopedTaskEnvironment instead of MessageLoop in content unit tests.

content/renderer/media_capture_from_element/html_audio_element_capturer_source_unittest.cc

Index: content/renderer/media_capture_from_element/html_audio_element_capturer_source_unittest.cc
diff --git a/content/renderer/media_capture_from_element/html_audio_element_capturer_source_unittest.cc b/content/renderer/media_capture_from_element/html_audio_element_capturer_source_unittest.cc
index ce3a94d8a5427ddde00a46b95d954abfb8f2a055..f5e908105177fadf517dd2a0c8af9ca68373a4b2 100644
--- a/content/renderer/media_capture_from_element/html_audio_element_capturer_source_unittest.cc
+++ b/content/renderer/media_capture_from_element/html_audio_element_capturer_source_unittest.cc
@@ -2,12 +2,13 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include "content/renderer/media_capture_from_element/html_audio_element_capturer_source.h"
 #include "base/memory/weak_ptr.h"
 #include "base/run_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "content/public/renderer/media_stream_audio_sink.h"
 #include "content/renderer/media/media_stream_audio_track.h"
-#include "content/renderer/media_capture_from_element/html_audio_element_capturer_source.h"
 #include "media/audio/null_audio_sink.h"
 #include "media/base/audio_parameters.h"
 #include "media/base/fake_audio_render_callback.h"
@@ -111,7 +112,7 @@ class HTMLAudioElementCapturerSourceTest : public testing::Test {
   }
 
  protected:
-  const base::MessageLoop message_loop_;
+  const base::test::ScopedTaskEnvironment scoped_task_environment_;
 
   blink::WebMediaStreamSource blink_audio_source_;
   blink::WebMediaStreamTrack blink_audio_track_;