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- On existing column headers, we should expose nothing for
AXColumnHeaderUIElements.
- For cells, we should expose only the column headers that are used for that cell,
not all of them as we currently do.
- The children of a column should be exposed (this is what actually fixes the VoiceOver issue in the bug).
- The children and visibleChildren should be the same.

Bug: 775167
Change-Id: Ief4d1ee80cd759250041e78e95e643030c14b361
Reviewed-on: https://chromium-review.googlesource.com/721420
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512707}