Critical security issues 'in the wild' only come up once every few months, but yes, when they do, the Chrome team has someone on duty 24 hours per day whose responsibility is to patch the code and do a release to all users within a matter of hours.

If you submit a security issue to Chrome, they actually have a tickbox on the webform to say "this issue is important enough to get someone out of bed for", and if you tick that box, it will actually wake someone at 3am to deal with it...