[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
1. move declearation of old_value into if closure
2. rewrite as
```
std::string key = it.key();
if (widget_storage_->EntryExists(key))
  continue;

...
3. question: you sure postmessage when the key does not exist?
```