Remove NOTIFICATION_WEB_CONTENTS_VISIBILITY_CHANGED from NewTabUI.

BUG=170921
TEST=everything still works

Review URL: https://codereview.chromium.org/26471006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227758 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227758