Bug 1828747 - Add a way to skip some shutdown CC logs. r=smaug

If MOZ_CC_LOG_SHUTDOWN_SKIP is set to a non-negative integer
value, then skip that many of the initial shutdown CC logs.
This is useful to reduce total log size if you know the
initial few logs aren't useful, because they are the largest.

Differential Revision: https://phabricator.services.mozilla.com/D175797