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Before this change, BoxReader could read syntax elements outside of a
box in cases where the MP4 was invalid. This isn't a security concern,
since the reads were still bounded by the buffer size, but it did make
it hard to interpret the later failures.

This CL shrinks |buf_size_| to match |box_size_| when the box size is
read. OOB reads will now result in Parse failures immediately.

Bug: 779321
Change-Id: I54be6ba3815d1502233b5e95d099ce9d9a54b3cc
Reviewed-on: https://chromium-review.googlesource.com/747080Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513000}