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Turns a possible UAF when trying to notify a deleted observer into a
CHECK(). This is achieved with minimal changes to ObserverList and an
adapter that gives WeakPtr-like semantics to an observer interface.

base::ObserverList<T>::Unchecked continues to use raw pointers that
are unchecked.

Bug: 842987, 808318
Change-Id: I8ab20845f4f6e1d2559490287731cea2dbf40d39
Reviewed-on: https://chromium-review.googlesource.com/1053338
Commit-Queue: Trent Apted <tapted@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584694}