Is chromium a good choice?
I believe so, yes!
The source code is IMO very well written, it's a really active project with a lot of work to do and is also interesting in many different ways. Obviously a browser is in itself just a combination of specific libraries, and thus Chromium gives you a nice entry to learn more about them and hopefully contribute evidently. But most importantly it has a big community, is sponsored by a big corporation and has many talented software engineers on its core team.
There are so many things to do, so you could even contribute things you know while learning stuff you don't.
I'd like to add; The choice of an open source project to join should be based on:
I only speak for myself here, but as much as I love learning more from too complex projects (file systems, RDBM etc) I find those projects to be less rewarding because of the overwhelming complexity. Try not to learn everything at once, take smaller steps and finish what you start rather than taking larger steps and give up.
Just my 2c - YMMV
In case you'd want to try Chromium out, here are links for the design documents:
The links above are all taken from the Chromium developer documentation, where even more details can be found.
Anyway, good luck finding a project that fits your needs!