Add Common Encryption support to BMFF, including subsample decryption.

media/mp4/box_definitions.h

Index: media/mp4/box_definitions.h
diff --git a/media/mp4/box_definitions.h b/media/mp4/box_definitions.h
index 7f7384b24d5340d0c480792c71d5798bd9a79f08..f1a3fc3f754c7e0ae28c41d7d5fba57e78bdeaaf 100644
--- a/media/mp4/box_definitions.h
+++ b/media/mp4/box_definitions.h
@@ -208,7 +208,9 @@ struct MEDIA_EXPORT SampleTable : Box {
   DECLARE_BOX_METHODS(SampleTable);
 
   // Media Source specific: we ignore many of the sub-boxes in this box,
-  // including some that are required to be present in the BMFF spec.
+  // including some that are required to be present in the BMFF spec. This
+  // includes the 'stts', 'stsc', and 'stco' boxes, which must contain no
+  // samples in order to be compliant files.
   SampleDescription description;
 };