Add a CHECK when an object tries to remove itself as an observer from NotificationService but no matching entry is found.  This is most likely an object being deleted on the wrong thread, and it'll lead to a crash later.

BUG=25354

Review URL: http://codereview.chromium.org/342091

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@30801 0039d316-1c4b-4281-b951-d872f2087c98