CLONE_NEWUSER / user namespaces is Linux-specific, yes. (Though see my other comment.)
I believe the Chromium sandbox mostly applies to things like renderers, where you can just hand it a buffer and a bunch of inputs and it'll effectively contain things like HTML parser bugs or libpng buffer overflows. I'm not totally sure if browsing itself is sandboxed.
My concern is that you can clearly browse and save files, so there is something that has access. Maybe it's much more secure than I thought, but I tend to be really skeptical towards programs of such length being perfectly secure.