GitLab

Changes:
Change shortcut Alt+Shift+N to toggling message center instead of only
showing message center.

BUG=376937

Change-Id: I44a062eb4d2117d642257241cbddf9a5feab97cf
Reviewed-on: https://chromium-review.googlesource.com/614726Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496563}