trunk-tracking: update from 3485 to 3497

* Remove NgxThreadSystem because there's now a SystemThreadSystem that
  has everything we need.
* fixes https://github.com/pagespeed/ngx_pagespeed/issues/364
  * the upstream code also changes the CHECKs to DCHECKS for `may_start_threads_` because they're not worth crashing a production server over.