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Per the spec DiscardPadding is a signed integer.  Sadly the parser
only supports UInt values at the moment.  To avoid refactoring the
world, we parse DiscardPadding as binary and have a one off parser.

A re-encoded bear-opus-end-trimming.webm will be landed separately
ahead of this change.

This patch also adds a check to ensure we never parse uint64 values
which will overflow an int64 container.

BUG=366750
TEST=Existing tests pass w/ DCHECK.

Review URL: https://codereview.chromium.org/251583003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266478 0039d316-1c4b-4281-b951-d872f2087c98