Fix race-condition in ct-embedded-flakiness-dashboard-tests.

There's no guarantee that the postMessage will arrive before
the requestAnimationFrame.

NOTRY=true

Review URL: https://codereview.chromium.org/397343006

git-svn-id: svn://svn.chromium.org/blink/trunk@178509 bbb929c8-8fbe-4397-9dbb-9b2b20218538