Unfortunately that’s an internal tool at Google and isn’t available more widely, as far as I know.  I assume someone has found the need and written an open source tool for visualizing protos, but I haven’t looked around.
One other idea may be to convert the proto to json (modulo format compatibility issues) and then use a json visualization tool.  Firefox has one built in.  I haven’t tried this though.
Truth be told, the tool in that post only handles 20MB files anyway, so it’s not that useful for debugging “real” models.  I frequently find myself having to use less and dig through the text representation of the proto by hand anyway.  I’m not sure about an open source tool to convert protos to text though.  Alas, it looks like the one I use isn’t open source either.  I think it should be trivial to do if you use the proto library for your language of your choice directly though.