Improvements to simulation hotloading before the SM upgrade, refs #4893.

SM45 will enforce property attributes described at https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/JSAPI_reference/Property_attributes. We thus cannot hotload properties with the DontDelete attribute.
Fix an oversight in the hotloading code.
Rename a confusing parameter.

Reviewed By: wraitii
Differential Revision: https://code.wildfiregames.com/D1844