Use FF_THREAD_SLICE threading mode in FFmpeg for MediaSource streams.

media/filters/ffmpeg_video_decoder.cc

Index: media/filters/ffmpeg_video_decoder.cc
diff --git a/media/filters/ffmpeg_video_decoder.cc b/media/filters/ffmpeg_video_decoder.cc
index ff14e89d3711196a713a3e562ee2d975fae758d9..1d784c73961c79081b2328d0aded98b5a4ae4902 100644
--- a/media/filters/ffmpeg_video_decoder.cc
+++ b/media/filters/ffmpeg_video_decoder.cc
@@ -55,8 +55,10 @@ static int GetThreadCount(AVCodecID codec_id) {
 }
 
 FFmpegVideoDecoder::FFmpegVideoDecoder(
-    const scoped_refptr<base::SingleThreadTaskRunner>& task_runner)
+    const scoped_refptr<base::SingleThreadTaskRunner>& task_runner,
+    ThreadingMode threading_mode)
     : task_runner_(task_runner),
+      threading_mode_(threading_mode),
       weak_factory_(this),
       state_(kUninitialized) {
 }
@@ -369,6 +371,8 @@ bool FFmpegVideoDecoder::ConfigureDecoder() {
   // for damaged macroblocks, and set our error detection sensitivity.
   codec_context_->error_concealment = FF_EC_GUESS_MVS | FF_EC_DEBLOCK;
   codec_context_->thread_count = GetThreadCount(codec_context_->codec_id);
+  codec_context_->thread_type =
+      threading_mode_ == THREADING_SLICE ? FF_THREAD_SLICE : FF_THREAD_FRAME;
   codec_context_->opaque = this;
   codec_context_->flags |= CODEC_FLAG_EMU_EDGE;
   codec_context_->get_buffer = GetVideoBufferImpl;