Infrastructure ('infra') refers to all of the servers and tools which the Chromium project depends on for development.
Most of these are maintained at Google by a dedicated team, but (almost) all of these are modifiable by anyone in the Chromium project.
To check out (most) of the Chromium Infra code use:
Join firstname.lastname@example.org for discussion of Chromium infrastructure related issues.
Most of the documentation is now at: https://chromium.googlesource.com/infra/infra/+/HEAD/doc/index.md.
Google employees can also find more information at the internal Chrome Infrastructure site.