Sorry for the confusion! There are a few paths forward here:
1) Link the accounts. I can do this for you, if you like. You'll also have to tell me which email address you want as the "primary" (the one that shows up in the UI).
2) Change your credentials. The message it printed about "the following addresses are currently registered" means that your credentials (~/.gitcookies) list your @chromium.org address. If you go to https://chromium.googlesource.com/new-password and sign in with your igalia account, then you'll be able to replace some of the lines in your current .gitcookies with the ones generated there, and then you will be pushing with the same account as is in your commit's committer field.

Personally, I'd do both. I think that your credentials should always match your git config user.email address, since that reduces confusion for everyone involved. But I can then also link your accounts, so that the system is more forgiving.

What would you like to do?

Ok, thank you for the details. I thought it was something like that,
but couldn't find the file that was storing my credentials.

Anyway I'd like to link my accounts and use rego@igalia.com as the primary one.
If you can do it, it'd be really nice. Thanks!

Done!