Refuse to render RSS as XML by treating the response as text/plain. This is

      somewhat unfortunate, but we need to do this until we have a built-in feed
previewer.

R=mal
BUG=21238

Review URL: http://codereview.chromium.org/201044

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@25608 0039d316-1c4b-4281-b951-d872f2087c98