StructurePortal's ticksToDecay property

```
> Object.keys(Game.getObjectById("656b2814e65fc62f3e736121"))
> id,room,pos,_structureType
```

"656b2814e65fc62f3e736121" is portal's id.
ticksToDecay property is not implemented???
https://docs.screeps.com/api/#StructurePortal.ticksToDecay
