The MP4 standard considers the udta box as a regular container box. Quicktime, however,
considers it as a container (of only leaf atoms) that can have a terminating 32 bit
integer with value 0. Since this breaks the principle of not having content in container
boxes, this CL considers the udta box as a leaf box that contains other boxes and does
the parsing manually.
Issue: #1315
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=117237255
