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This CL creates a base class for notification builder mac and extends it
to build UNNotifications. It also groups shared methods between the NPBs
for macOS platforms together to avoid redundancy.

Bug: 1129987, 1129988
Change-Id: I41ef920aafed94a0ac8e9851c5884112f0a30aff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2418446Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Richard Knoll <knollr@chromium.org>
Commit-Queue: Mohamed Adel <adelm@google.com>
Cr-Commit-Position: refs/heads/master@{#809361}