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This implements notification suspension if a site is paused. The
SuspensionTracker notifies the new NotificationSuspender of changes. The
NotificationPlatformBridge depends on the NotificationSuspender to check
if a notification is allowed to be displayed.

Bug: 949475
Change-Id: Id66bfd1479a7e77f34f6322d4432c5103d17d9ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628753
Commit-Queue: Richard Knoll <knollr@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Patrick Noland <pnoland@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666393}