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Double adding and removing of an observer hits DCHECK. This patch prevents that.
Bug: none
Change-Id: I289cadc8356783abdfac3fee6d2e3a57d620cc21

Change-Id: I384ea7ee20a98ac03f1661f48cc599d6f8acb8c4
Reviewed-on: https://chromium-review.googlesource.com/583958
Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Reviewed-by: David Reveman <reveman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490942}