GitLab

When a modal dialog appears, GetChildCount and ChildAtIndex just
consider the modal dialog and skip other children. Since
GetIndexInParent compares itself with all siblings to get the
index, if the parent has a modal dialog, it couldn't find anything
matched.

This CL returns nullopt on GetIndexInParent if its parent has a
modal dialog.

AX-Relnotes: n/a.
Bug: 1104282
Change-Id: I3c8b45fe96aa188fd29aa63bc5fd4b1fc0bac476
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2299664
Commit-Queue: Julie Kim <jkim@igalia.com>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790734}