[Tizen ]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
This chunk of code is in <code>clear</code> function. If the key still exist, it means it is read only and can't be removed. So if the key does not exist, it means this entry has been removed. We should notify other documents.