Fire storage event at window when storageArea change

According to the spec http://dev.w3.org/html5/webstorage/#dom-storage-clear,
'When the setItem(), removeItem(), and clear() methods are invoked, events
are fired on the Window objects'. This change will implement this feature.

BUG=XWALK-2991
```
if (!window.CustomEvent)
  return;

...
```

And do we need check window.CustomEvent? I think it is always available.