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In current code, notification may be removed before animation which is
triggered by the previous operation on MessagePopupCollection ends. As
result, when a new notification with the same ID is created, calling
MessagePopupCollection::Update will not update the popup's content. Then
the new notification popup fails to show. The CL fixes this bug.

Test: message_center_unittests
Bug: 921402
Change-Id: I56743b5e17487d3f5bf52ef934b0a53d8f6471d0
Reviewed-on: https://chromium-review.googlesource.com/c/1422738Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626116}