Make supported-xml-content-types.html tests faster.

To try to address Windows test slowness, switch this test over to
async XHR requests + split it up into chunks.

R=mkwst@chromium.org
BUG=

Review URL: https://codereview.chromium.org/198893002

git-svn-id: svn://svn.chromium.org/blink/trunk@169317 bbb929c8-8fbe-4397-9dbb-9b2b20218538