Remove no_size_t_to_int_warning in //ui/message_center

Bug: 167187
Change-Id: Ie7b6c5714567d8486fcc6ce16f17cc418ca6ac30
Reviewed-on: https://chromium-review.googlesource.com/c/1393332
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619463}