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 pendingIntents are 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.

Platforms other than Android keeps empty implementaion for notification related
interfaces.

TEST=XWalkCoreTest org.xwalk.core.xwview.test.WebNotificationTest
BUG=https://crosswalk-project.org/jira/browse/XWALK-94
Merge pull request #1470 from wang16/w3c_notification

[Android] Implement W3C notification API