Gardening M38: Contents client now needs to own NotificationDelegate

According to upstream commit e4c9ef21d. Crosswalk on Android need
similar approach to own NotificationDelegate for each web
notification.

Use ScopedPtrHashMap for it.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2404
Merge pull request #2361 from wang16/next

Gardening M38: Contents client now needs to own NotificationDelegate