Separate listeting for notification data from subscribing.

Manage subscriptions depending on if file is visible and also active.

R=brianwilkerson@google.com
BUG=

Review URL: https://codereview.chromium.org//645403005

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41273 260f80e4-7a28-3924-810f-c04153c831b5