GitLab

I speculate that WebAXObject::IsDetached() may not be as reliable
as Document::IsDetached().

Also, add a SECURITY_CHECK in the place where I suspect the bad
access is happening.

BUG=722801
R=dmazzoni@chromium.org,skobes@chromium.org

Review-Url: https://codereview.chromium.org/2892283002
Cr-Commit-Position: refs/heads/master@{#473448}