Support for `chrome`/`chrome_extensions` externs

Hey there,

We have some interest in building a Chrome extension with the elemental2/J2CL stack. However, we can't yet use elemental2 to great effect because there isn't a `chrome` and/or `chrome_extension` package that brings those extern files into Java.

I know this project is quite early, but we wanted to file this to track progress against this goal. We are also happy to propose PRs to introduce this functionality.

Thank you for this awesome project and we're excited to help make it better how we can