Allow media track switching.

media/renderers/default_renderer_factory.h

Index: media/renderers/default_renderer_factory.h
diff --git a/media/renderers/default_renderer_factory.h b/media/renderers/default_renderer_factory.h
index 7fe4a5ee6422c9be7eb42c975d9231816f181b8d..f9f203f79a5f66c3988eb6b57f2542ebbf172120 100644
--- a/media/renderers/default_renderer_factory.h
+++ b/media/renderers/default_renderer_factory.h
@@ -23,6 +23,9 @@ class MediaLog;
 class VideoDecoder;
 class VideoRendererSink;
 
+using CreateAudioDecodersCB = base::Callback<ScopedVector<AudioDecoder>()>;
+using CreateVideoDecodersCB = base::Callback<ScopedVector<VideoDecoder>()>;
+
 // The default factory class for creating RendererImpl.
 class MEDIA_EXPORT DefaultRendererFactory : public RendererFactory {
  public: