This is required as part of the Accessibility Object Model
phase 2 spec.

Originally landed: r496801
Reverted: r498006 due to:
* crbug.com/759379 - fixed by r498167
* crbug.com/758510 - fixed by patch set 2
    (null check in AXObjectCacheImpl.cpp)

Bug: 746524,759379,758510
Change-Id: I6acf62366da7e6700dfa43e3c9cf10b4c6a966de
Tbr: haraken,mlamouri
Reviewed-on: https://chromium-review.googlesource.com/641983Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#498367}