dom-ui settings: Stop observing an existing BrowserDataRemover before creating a new one.

      BUG=81916
TEST=Follow repro setps in bug report, verify that crash no longer occurs.
Review URL: http://codereview.chromium.org/7050031

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@86146 0039d316-1c4b-4281-b951-d872f2087c98