Firstly, while I'm not very familiar with Erlang or its runtime, afaik Erlang processes are not actual OS processes.
Secondly threads were originally created as a cheaper alternative because spawning processes was too expensive (on Windows). The original reason for Chrome using processes was that processes gives you memory protection. But I don't know how much it really helped them in the end, because they quickly noticed that using one process per tab doesn't scale. Thus Chrome (at least originally) limited the maximum number of processes to ten.
Secondly threads were originally created as a cheaper alternative because spawning processes was too expensive (on Windows). The original reason for Chrome using processes was that processes gives you memory protection. But I don't know how much it really helped them in the end, because they quickly noticed that using one process per tab doesn't scale. Thus Chrome (at least originally) limited the maximum number of processes to ten.