NearHeapLimitCallback Not Getting Called

[joelrs...@gmail.com]

Hi, I have a use case that is intending to use the near heap limit callback, but i'm not seeing the near heap limit callback getting called. For simple javascript such as:

```
var large_arr = [];
while(true) {
    large_arr.push("Execute order 66");
 }
```

I am setting the max_old_generation_size to 3mb in this example. I see GC cycles using the prolog/epilog callbacks, but never the near neap limit callback and it goes far past 3mb. Any tips on what limit's/callbacks I should be setting or using to interrupt this type of script promptly would be great, thanks!