Make all the readonly attributes in Device Capabilities valid.

Bug=XWALK-2162
@halton , I found it failed at this line `createConstClone(data)` for listener. It should be wrong. I try hard to understand the `EventTargetPrototype`'s structure but can not understand totally. I can not find where the real recieved data is. Try to add `createConstClone` in line 190, 230 or 245 and all are failed. Could you please have a look at this?