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These fields are added as requested by ChromeVox's team to tell
ChromeVox user the true number of candidates and the current selection's
index.

Bug: 1074158
Change-Id: I697f96fe955fd5dc151543d47b26c605060f0a37
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2162501
Commit-Queue: My Nguyen <myy@chromium.org>
Reviewed-by: Keith Lee <keithlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762794}