Prevent people from adding mime type to supported_non_image_types that will make it render as HTML. This is not the fix for bug, but to prevent people from making the same mistake again.

      BUG=38105
TEST=None

Review URL: http://codereview.chromium.org/1617013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@44143 0039d316-1c4b-4281-b951-d872f2087c98