Bug 1504060 - Log instead of asserting that webm samples do not have timestamps > segment druation. r=alwu

We're able to hit this assertion in the wild due to bad muxers. As such, replace
the assert with a log. If a sample has a time stamp > the segment duration, use
that instead of the duration for calculating our next time stamp. Use an
explicit int64_t type in the signature for our next time stamp calculation
as the logging explicitly expects an int64_t (makes it harder to change the
types involved and footgunning by having a wrong formatter in the logs).

Differential Revision: https://phabricator.services.mozilla.com/D21717

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extra : moz-landing-system : lando