[LayoutNG] Disallow hit-testing NG list markers

List markers are not selectalbe. Before this change, it still
hits in |LayoutNGMixin::PositionForPoint()| as atomic inline,
which leads to crashes.

Change-Id: I4563c493417cf72900f9da48a052f564fb5f0369
Bug: 964813, 964814, 964925
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1619593
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661376}
6 files changed