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This is a follow up of [1] and [2], where droger asked to move
away from using "unchecked" ObserverLists.

[1] https://crrev.com/c/1448473
[2] https://crrev.com/c/1448754

BUG=927341,927343

Change-Id: I5d91a6af2f51af50eb3a803e908c79c8f1789bc4
Reviewed-on: https://chromium-review.googlesource.com/c/1452818
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Auto-Submit: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629103}