be more explicit about which mimetypes to skip so svg support files aren't skipped

BUG=404846

R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/485183004

git-svn-id: svn://svn.chromium.org/blink/trunk@181358 bbb929c8-8fbe-4397-9dbb-9b2b20218538