Mark dont-load-unknown-type.html as failing on Mac

The MIME type of the JavaScript file in this test has changed. It's the
only failing test on this, so I'm disabling it until Eric can take a look.

TBR=jchaffraix@chromium.org, eseidel@chromium.org
BUG=380649

Review URL: https://codereview.chromium.org/316903002

git-svn-id: svn://svn.chromium.org/blink/trunk@175475 bbb929c8-8fbe-4397-9dbb-9b2b20218538