Fix crash in blink::AXSelection when selection is on shadow root

Bug: 998551
Change-Id: Ic8356adc9fa24752812bbaeca2aeb97aa4e2a4f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1791142
Commit-Queue: Jessica Tallon <jtallon@igalia.com>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695642}
2 files changed