CQ, aka Commit Queue, verifies patches before they are submitted. As of Dec 2018, it works only with patches on Gerrit code review.
Trivia: CQ, despite its name, is not actually a queue, but a set. CQ verifies each CL independently of other unrelated CLs and on success CQ submits these CLs using Gerrit API in arbitrary order.
As of Dec 2018, Commit Queue consists of two parts:
Daemon, whose code is currently in infra_internal, but should be open sourced per https://crbug.com/696648. Since daemon is currently in Python, it may end up in different repository, so follow the bug if you care.
Config validator GAE app, which is already written in Go, but is still located in infra_internal repo.
This is a GAE app, currently under construction, to replicate config validator functionality (tracking bug https://crbug.com/884556). With time, new CQ features and some chunks of Daemon will be added/rewritten in Go here.