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This is preferred over unchecked observers.

Bug: b:120097552
Change-Id: Id44822dce375e3400baa336fee3154ad99d18d7f
Reviewed-on: https://chromium-review.googlesource.com/c/1354569
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611999}