Document purpose of low_delay flag in VideoDecoder interface.

media/base/video_decoder.h

Index: media/base/video_decoder.h
diff --git a/media/base/video_decoder.h b/media/base/video_decoder.h
index d7e7a791de3d168668476cbdbd73aae22dd90ef4..bba581dc73fd343085070344a22e573191106931 100644
--- a/media/base/video_decoder.h
+++ b/media/base/video_decoder.h
@@ -55,6 +55,12 @@ class MEDIA_EXPORT VideoDecoder {
   // |status_cb| upon completion. |output_cb| is called for each output frame
   // decoded by Decode().
   //
+  // If |low_delay| is true then the decoder is not allowed to queue frames,
+  // except for out-of-order frames, i.e. if the next frame can be returned it
+  // must be returned without waiting for Decode() to be called again.
+  // Initialization should fail if |low_delay| is true and the decoder cannot
+  // satisfy the requirements above.
+  //
   // Note:
   // 1) The VideoDecoder will be reinitialized if it was initialized before.
   //    Upon reinitialization, all internal buffered frames will be dropped.