What does "CL" mean in a commit message? What does it stand for?

It means Change List.

Create a change list (CL)

Creating a change in git is really just creating a branch.

http://www.chromium.org/developers/contributing-code

And the code review system there are using: https://code.google.com/p/rietveld/

It seems to be Google-speak. I don't work for Google so I can only guess, but I'd guess it means "changelog", in the meaning of "a small set of commits being considered as a single unit for merging", much like a Github PR (pull request). It shows up here and here (search in the page for "CL created") with that same usage, and it shows up all over the place in the Angular issues.

If you're more familiar with Github-speak than Google-speak, mentally substitute "PR" for "CL" and I think you'll have the right idea. :-)

CL: Stands for “changelist,” which means one self-contained change that has been submitted to version control or which is undergoing code review. Other organizations often call this a “change” or a “patch.”

*from the Terminology of the Google Engineering Practices Documentation that can be found here: https://google.github.io/eng-practices/