GitLab

Calling ResetRepositionSession during UpdateNotificationSize can end up
deleting notification items. This caused two crashes. One in
AddNotification, which calls onItemUpdated, which, without this change,
can set item_ to null. The other is a crash in
ShouldUpdateControlButtonsColor which does not check if item_ is null
since it is called from a context where item_ should not be null.

BUG=714493
BUG=714032

Review-Url: https://codereview.chromium.org/2833403002
Cr-Commit-Position: refs/heads/master@{#466914}