Revert of Replace FFmpegDemuxer thread per element with base::TaskScheduler.

media/filters/ffmpeg_demuxer_unittest.cc

Index: media/filters/ffmpeg_demuxer_unittest.cc
diff --git a/media/filters/ffmpeg_demuxer_unittest.cc b/media/filters/ffmpeg_demuxer_unittest.cc
index d0a7d1a2565d94d598310d1952984f73171272b7..92766f1cdcfe4db048b6ddf9dc73998d72feb60d 100644
--- a/media/filters/ffmpeg_demuxer_unittest.cc
+++ b/media/filters/ffmpeg_demuxer_unittest.cc
@@ -18,7 +18,6 @@
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/test/mock_callback.h"
-#include "base/test/scoped_task_scheduler.h"
 #include "base/threading/thread.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "media/base/decrypt_config.h"
@@ -86,8 +85,6 @@
   virtual ~FFmpegDemuxerTest() {
     if (demuxer_)
       demuxer_->Stop();
-    demuxer_.reset();
-    base::RunLoop().RunUntilIdle();
   }
 
   void CreateDemuxer(const std::string& name) {
@@ -104,7 +101,7 @@
         &FFmpegDemuxerTest::OnMediaTracksUpdated, base::Unretained(this));
 
     demuxer_.reset(new FFmpegDemuxer(
-        base::ThreadTaskRunnerHandle::Get(), data_source_.get(),
+        message_loop_.task_runner(), data_source_.get(),
         encrypted_media_init_data_cb, tracks_updated_cb, new MediaLog()));
   }
 
@@ -191,8 +188,9 @@
                 buffer->discard_padding().first);
       EXPECT_EQ(read_expectation.is_key_frame, buffer->is_key_frame());
     }
+    DCHECK_EQ(&message_loop_, base::MessageLoop::current());
     OnReadDoneCalled(read_expectation.size, read_expectation.timestamp_us);
-    base::ThreadTaskRunnerHandle::Get()->PostTask(
+    message_loop_.task_runner()->PostTask(
         FROM_HERE, base::MessageLoop::QuitWhenIdleClosure());
   }
 
@@ -241,12 +239,11 @@
   }
 
   // Fixture members.
-
-  base::test::ScopedTaskScheduler task_scheduler_;
   std::unique_ptr<FileDataSource> data_source_;
   std::unique_ptr<FFmpegDemuxer> demuxer_;
   StrictMock<MockDemuxerHost> host_;
   std::unique_ptr<MediaTracks> media_tracks_;
+  base::MessageLoop message_loop_;
 
   AVFormatContext* format_context() {
     return demuxer_->glue_->format_context();