Replace FFmpegDemuxer thread per element with base::TaskScheduler.

media/base/demuxer_perftest.cc

Index: media/base/demuxer_perftest.cc
diff --git a/media/base/demuxer_perftest.cc b/media/base/demuxer_perftest.cc
index adfde41002e92b4e49a849cf8b7197f41fa76f32..d00447519d105958efb9457566f66b23cd41e697 100644
--- a/media/base/demuxer_perftest.cc
+++ b/media/base/demuxer_perftest.cc
@@ -12,6 +12,7 @@
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/strings/string_number_conversions.h"
+#include "base/test/scoped_task_scheduler.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
@@ -185,6 +186,7 @@ static void RunDemuxerBenchmark(const std::string& filename) {
     FileDataSource data_source;
     ASSERT_TRUE(data_source.Initialize(file_path));
 
+    base::test::ScopedTaskScheduler scoped_task_scheduler(&message_loop);
     Demuxer::EncryptedMediaInitDataCB encrypted_media_init_data_cb =
         base::Bind(&OnEncryptedMediaInitData);
     Demuxer::MediaTracksUpdatedCB tracks_updated_cb =