The issue could lead to the MPD attribute mediaPresentationDuration being
wrongly generated for WebM streams with a duration longer than INT32_MAX.
The root-cause was that StreamInfo::set_duration() accepted an int instead of a
uint64_t. This seems like a pure typo, since StreamInfo already uses a uint64_t
internally for representing duration.
Additionally, for XML schema correctness, convert ISO-639-2 language
tags (three-letter codes) to ISO-639-1 tags (two-letter codes) when
possible. This follows BCP-47, which says to always use the shortest
language tag when there are multiple possibilities.
b/18613148
Change-Id: I120fb7b42ac4da5feb4ca046bba93d9e58acd7a9