(This page is intended for Certificate Authorities who wish to know about Chromium's certificate revocation behaviour.)
CRLSets (background) are primarily a means by which Chrome can quickly block certificates in emergency situations. As a secondary function they can also contain some number of non-emergency revocations. These latter revocations are obtained by crawling CRLs published by CAs.
Online (i.e. OCSP and CRL) checks are not, generally, performed by Chrome. They can be enabled by policy and, in some cases, the underlying system certificate library always performs these checks no matter what Chromium does.
The Chromium source code that implements CRLSets is, of course, public. But the process by which they are generated is not.
We maintain an internal list of crawled CRLs which are intended to cover intermediate revocations. The CRLs from that set go to make up the published CRLSet. CRLs on the list are fetched infrequently (at most once every few hours) and verified against the correct signing certificate for that CRL.
The current CRLSet can be fetched and dumped out using the code at https://github.com/agl/crlset-tools.
The version of CRLSet being used by Chrome can be inspected by navigating to chrome://components: