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Previously, each OverlayRequestCoordinator had an associated
OverlayRequestMediator that also supported the same request
type.  However, with the introduction of infobar overlays,
we will have a single OverlayRequestCoordinator managing the
UI, but separate mediators that support only a specific
request config type used to configure the shared consumer
protocol.

This CL adds class properties to the coordinator and mediator
interfaces that return OverlayRequestSupports rather than
keeping |+supportsRequest:|.  This allows each class to
specify its request support separately without the need to
introduce an additional |+supportRequest:| to the mediator.

Bug: none
Change-Id: I990467855d6cd1193db9d70e68ce412866128f74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1981198
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727525}