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Display observers have historically caused problems on Chrome OS.
They are not performance critical, so converted from UncheckedObserver
to checked observer.

Bug: 842987
Change-Id: I803afb4c6b5a2ef8e2bf1652183a8835243d07ec
Reviewed-on: https://chromium-review.googlesource.com/c/1263338Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597374}