Rewrite rel-import-to-style.html to kill the flakiness.

BUG=370490
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/278643002

git-svn-id: svn://svn.chromium.org/blink/trunk@173668 bbb929c8-8fbe-4397-9dbb-9b2b20218538