Only permit URL-based MIME type sniffing if the proposed type was empty or

      generic. See the bug for details on the stats we collected to determine that
it's not a major compat issue.

BUG=75070
Review URL: http://codereview.chromium.org/7397009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@92880 0039d316-1c4b-4281-b951-d872f2087c98