Chromium development is still very much active on Google Code, which will turn read-only on August 25th 2015.
What are the plans to move Chromium development?
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If you click on the link "this post", you will see this page:
It contains more information about the process, including this text:
As previously announced, most of Google Code will become read-only on August 25th, 2015.
We are still working with Google-affiliated projects (e.g., /p/chromium and /p/android) to get them migrated to other tools and make their projects read-only by the end of the year. These projects will remain read-write until we can transition all their data off of Google Code. (my emphasis)
So it says that you will be able to work on Chromium (and Android) in read-write mode until Google decides where to migrate them to some other tool chosen by Google (e.g. GitHub).
When a decision to migrate is made, it will probably be announced on that same page.