Make that function static, so that it can be used for these functions
without slowly having to obtain the ScriptInterface instance using
GetScriptInterfaceAndCBData just to get the JSContext again.
Remove few redundant conversions for CreateObject arguments.
Differential Revision: https://code.wildfiregames.com/D2128
Tested on: gcc 9.1.0, clang 8.0.1, Jenkins
Tedious performance testing in: D2128, D2127
This was SVN commit r22894.