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If the lock screen notification is disabled, NotificationHiddenView had
a leak. This CL fixes that.

Bug: None
Change-Id: I0ddb2a37777849c44ac19e4398ea88369fcb4a08
Reviewed-on: https://chromium-review.googlesource.com/1208694Reviewed-by: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589113}