Fix crash by MockPagePopupDriver.

We assumes no events are dispatched in
PickerIndicatorElement::openPopup. However they are dispatched with
MockPagePopupDriver and we should avoid ClusterFuzz false alarms.

BUG=379467
TEST=none; No behavior changes in production.

Review URL: https://codereview.chromium.org/309953002

git-svn-id: svn://svn.chromium.org/blink/trunk@175406 bbb929c8-8fbe-4397-9dbb-9b2b20218538