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A crash might happen when an observer to MessageCenter removes notification in handler, because future observer couldn't access to the notification. This patch fixes the issue by adding a queue and defering requests during itaration.

Bug: 762803
Test: Added test passes
Change-Id: Id2c9a34602e14869c9b6db7d6dd7cff94b0089f0
Reviewed-on: https://chromium-review.googlesource.com/676624Reviewed-by: Eliot Courtney <edcourtney@chromium.org>
Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508609}