Fix DCHECK failure on reload of devices page

The devices page fails a DCHECK in NotificationRegistrar due to the fact that
the handler object is kept on reload and thus duplicate notifications are
registered. Fux this by removing all registered notifications before adding the
notifications anew.

BUG=

Review URL: https://codereview.chromium.org/25767004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226731 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226731