Add a checker to help find bugs in an AXTreeSource.

Bug: 911945
Change-Id: I64af0f02733e46bf1cd6df73d4ed0735d662c1b3
Reviewed-on: https://chromium-review.googlesource.com/c/1364159
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615973}
6 files changed