It is explained at the top-level document: https://google.github.io/eng-practices/#terminology
Re:How to do a code review
by throwaway_2047 on Friday September 06, 2019 @02:49AM
(#20893599)
Agree. I only realise it when reading through comments in HN
However, you can tell this was written by an engineer. The acronym CL is used about a hundred times. There is not a single page where the first usage of the acronym explains the full term.