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Since there many be multiple objects interested in data changes, it's
more natural to use an observer pattern instead.

Change-Id: Ib7e291626bf3f3dce23a73ebc329f96bc8f9deb9
Reviewed-on: https://chromium-review.googlesource.com/c/1293990
Commit-Queue: John Wu <jzw@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601662}