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This CL adds an interface to process user events. The glue layer of
the scheduler will implement this interface in a following CL. Also
the guid for the impression data is added to identify the specific
notification that the user interacted with.

This is a temporary solution and later we may convert to use
NotificationDisplayService at some point.

Bug: 930968
Change-Id: I6b5665fe30bec048a0c986bfad2a78ff02f38de8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1582428Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654631}