[whatwg] JSON encoding

[Douglas Crockford]

 > I think the hazard is very real and very serious.
 > I don't see any additional expressiveness enabled by toJSONString.

No one is more concerned about the security of the platform than I am.
There isn't a security issue here. The hazard, if there is one, is
certainly not serious.

This is a convenience issue. Having toJSONString as a builtin is a convenience, 
removing the need to load json.js.