Specify a maximal depth of backtracking for RegExp

[annevk]

JSC currently enforces a limit. Other engines don't, but their host does enforce a limit for <input type=pattern>. Consider defining this limit directly in ECMAScript instead.

If this limit is not defined directly in ECMAScript, we would still need a host hook for it, at least for <input type=pattern>, though potentially we could instead enforce it in HTML for the web platform in general.

See whatwg/html#9469 for context.

cc @schuay @mathiasbynens @vinhill @msaboff