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This introduces a new NotificationPlatformBridgeDelegator that is
responsible for selecting the correct platform bridge for notification
events.

Bug: None
Change-Id: I2aaff169887e281ab46fc51b8297a8cdf610ed9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2401161Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Richard Knoll <knollr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806945}