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This CL adds NotificationAccessManager::AttemptNotificationSetup(),
which starts the flow for enabling notification access. Note that the
real implementation for this flow has not yet been completed. However,
the new code added to FakeNotificationAccessManager will unblock UI
work which needs this code in place to write unit tests.

A follow-up CL will actually implement the flow itself.

Bug: 1106937
Change-Id: I2d0ccedbe758252857ec2238a294b8a2a1f31eeb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354657
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Josh Nohle <nohle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797949}