Sandboxes always default to JSVERSION_DEFAULT in the browser. But XPCShell sets
up a ContextCallback that does JS_SetVersion(cx, JSVERSION_LATEST) on every
context that gets created, including the ephemerial Sandbox JSContexts. Since
httpd.js runs in xpcshell and evaluates SJS in a sandbox, we've (somewhat
accidentally) supported JSVERSION_LATEST in SJS, which certain SJS files have
taken advantage of. Let's continue to support it explicitly.