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This refactors the macOS AlertDispatcher interface to no longer require
an instance of NSUserNotificationCenter to be passed in. We expect the
caller to query their local NotificationCenter instead and merge the
results. This allows us to use the same interface in a future CL for the
UNNotification API.

Bug: 1127306
Change-Id: I207eaf454ed86a7636725949269de5fee780167b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2640119Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Commit-Queue: Richard Knoll <knollr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845609}