160M is the value used by the Mozilla/SpiderMonkey JSShell project, so I'm
rationalizing that as an acceptable default. Of course, it would be good
to optimize memory usage in JS scripts, so we'll work on that too. So, if
you're a JS developer, best to play with lower values here to see if/where
you're being potentially wasteful in your memory usage, but most sysops
just want everything "to work".