Replace trampolining in FFmpegDemuxer with explicit thread calling convention checks.

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 9ce865938d8352ab565492e5a631b81354a7ea1e..a43a5a02accab08cf738ccf256553ac487f4d407 100644
--- a/media/filters/ffmpeg_demuxer_unittest.cc
+++ b/media/filters/ffmpeg_demuxer_unittest.cc
@@ -531,6 +531,7 @@ TEST_F(FFmpegDemuxerTest, DisableAudioStream) {
   // stream without requiring the message loop to read data.
   bool got_eos_buffer = false;
   audio->Read(base::Bind(&EosOnReadDone, &got_eos_buffer));
+  message_loop_.RunUntilIdle();
   EXPECT_TRUE(got_eos_buffer);
 }