Fix null dereference on local frame

Clusterfuzz identified a null pointer dereference in
ComputedAccessibleNode. The local frame is requested from the document
reference stored on the element, which can be null.

BUG=919310

Change-Id: I9deb28a0dea042ffc96dba7ce8ddd9efeeab2bd8
Reviewed-on: https://chromium-review.googlesource.com/c/1457696
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636049}
2 files changed