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This view provides the UI for in-Assistant notifications. Currently the
labels/buttons for this view are stubbed in but a follow up CL will wire
this view up to actual mojom::AssistantNotifications.

Screenshot: http://shortn/_MYjBlqDoku

Bug: b:118654460
Change-Id: I1a436c33466aff9db0c92595a988170c99456e0f
Reviewed-on: https://chromium-review.googlesource.com/c/1453547
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630144}