These tests do things like:

    eventSender.keyDown("rightArrow");
    expectTrue(document.scrollingElement.scrollLeft == (window.innerWidth * scale), "arrow key div scrolled to second div.");

With UI-side compositing, arrow handling hits AsyncScrollingCoordinator::requestScrollPositionUpdate(), which bounces to the UI process, and then IPCs back to the web process via AsyncScrollingCoordinator::reconcileScrollingState() which updates the ScrollableArea's scroll position, which is then read by `scrollLeft`.

So the tests need to wait for a presentation update after dispatching the key.