Override notification related interface from content

There are four methods in content browser client which are for
client to implement to support web notification.
Override those apis with empty function bodies first.
[Android] Implement W3C WebNotification API

W3C web notification API is defined at http://www.w3.org/TR/notifications/.
On Android, web notifications are well matched to system notifications.
To invoke js callbacks when system notification got clicked or canceled,
two pendingIntent is binded to the notification.
Since Android only keeps one pendingIntent for specified Context/Class/Category,
the unique id of the notification is used as the Category of the pendingIntents.

TODO(wang16): The icon field of web notification is not supported yet, the
system notification will use the app's icon currently. It's because Notifications
on Android only accept resource id for icon. But it accepts Bitmap for big icon,
will implement async fetch for the icon url in future.

TEST=XWalkCoreTest org.xwalk.core.xwview.test.WebNotificationTest
BUG=https://crosswalk-project.org/jira/browse/XWALK-94