http/tests/security/cross-origin-css.html is flaky

BUG=340206
TBR=caseq

Review URL: https://codereview.chromium.org/147573004

git-svn-id: svn://svn.chromium.org/blink/trunk@166299 bbb929c8-8fbe-4397-9dbb-9b2b20218538