[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
You should check the getxxx before execute next getxxx, suggested code as(maybe not compiltable, you can do that)
```
base::DictionaryValue* dict;
msg->GetAsDictionary(&dict);
DCHECK(dictionary);

std::string key;
std::string value;
if (!dict->GetString(kPreferencesItemKey, &key) &&
    widget_storage_->GetValueByKey(key, &value))
    return new base::StringValue(value);

return new base::StringValue(std::string());
```