function onKeyDown() {
        /** Direct Check C. See explanation in {@link recordIfFinished} */
        recordIfFinished();
        performance.mark('inputlatency/start');
        performance.mark('keydown/start');
        state.keydown = 1 /* EventPhase.InProgress */;
        queueMicrotask(markKeyDownEnd);
    }
Record the start of the keydown event.