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Add mojo interface for communication between ash and lacros. For now,
just add the fields needed for simple text notifications.

Notifications are handled in a new class MessageCenterAsh. It lives
in //chrome/browser/chromeos/crosapi instead of //c/b/notifications
for consistency with other crosapi implementations.

On mojo disconnect (e.g. lacros-chrome crash) all notifications
created by lacros are removed.

I'm working on tests now. They are not in this CL because this CL
is getting too large.

Test: Spawn basic notifications from test app Notifications Galore
      in chrome/test/data/extensions/api_test/notifications/galore/
Bug: 1113889

Change-Id: I6363b0e7568756b9087f089651a7fdc6d0901902
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343849Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Jun Mukai <mukai@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797969}