Fix crash with rtc_event_log in video_loopback

Because RtcEventLog is created and stopped in different threads,
SequencedTaskChecker causes failure at the end of a test.

Bug: none
Change-Id: Ibaec3162eedebd180b101ec46a171efee5fe667e
Reviewed-on: https://webrtc-review.googlesource.com/5401
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20078}
diff --git a/video/video_quality_test.cc b/video/video_quality_test.cc
index 2bfe0e3..2526ac2 100644
--- a/video/video_quality_test.cc
+++ b/video/video_quality_test.cc
@@ -1912,6 +1912,8 @@
 
   analyzer->Wait();
 
+  event_log_->StopLogging();
+
   task_queue_.SendTask([&]() {
     for (std::unique_ptr<test::VideoCapturer>& video_caputurer :
          thumbnail_capturers_)
@@ -1929,7 +1931,6 @@
     DestroyStreams();
     DestroyThumbnailStreams();
 
-    event_log_->StopLogging();
     if (graph_data_output_file)
       fclose(graph_data_output_file);