Mocha shouldn't wait for an animation frame

When we run the tests headless, we never actually put up frames, which means we
never generate animation frames. In this mode, we might want to generate fake
animation frames (like DRT does), but for now just use a timeout to make the
tests work again.

R=rafaelw@chromium.org, esprehn@chromium.org

Review URL: https://codereview.chromium.org/758113003