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This makes screen reader users aware that we've created bookmarks.

R=scottchen@chromium.org

Bug: 881937
Change-Id: I36f246768d6dfbf631860d1da0c043a6b7d20781
Reviewed-on: https://chromium-review.googlesource.com/c/1302494
Commit-Queue: Hector Carmona <hcarmona@chromium.org>
Reviewed-by: Scott Chen <scottchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604060}