Clusterfuzz identified a null dereference on AXLayoutObject.

Bug: 970217
Change-Id: I269ba9bfce1d07e6af48a11bd9080279400ca965
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1666576
Commit-Queue: Meredith Lane <meredithl@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671984}