Recent failures in bug 1204281 demonstrate that linux64-ccov non-e10s jsreftests sometimes run longer than 60 minutes, causing a task timeout. Timeouts started around June 8 -- https://treeherder.mozilla.org/#/jobs?repo=mozilla-central&revision=e61060be36424240058f8bef4c5597f401bc8b7e&filter-searchStr=ccov+jsreftest -- but those tasks were running in 50-55 minutes a week prior, so I think this is simply organic growth, best addressed with more test chunks.

This reduces the job times to between 30 and 45 minutes each: https://treeherder.mozilla.org/#/jobs?repo=try&revision=f5ddd97572f5144b648d6fbf59c264096f45e924

Pushed by gbrown@mozilla.com: https://hg.mozilla.org/integration/mozilla-inbound/rev/63cc5b52b0bf Increase linux64-ccov jsreftest chunks; r=jmaher