Deflake fast/canvas/webgl/premultiplyalpha-test.html.

It's an async test, so it should use js-test's async code instead
of directly calling waitUntilDone/notifyDone.

BUG=321787
TBR=kbr@chromium.org

Review URL: https://codereview.chromium.org/92823002

git-svn-id: svn://svn.chromium.org/blink/trunk@162801 bbb929c8-8fbe-4397-9dbb-9b2b20218538