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This class was unnecessary, as the same functionality can be
implemented much more easily by observing OverlayPresenter from
BadgeMediator, which acts as the InfobarBadgeTabHelper's
delegate.  This implementation will be added in a follow-up
CL.

This CL replaces the browser agent's instantiation with an infobar-
specific BrowserAgent creation helper function that will set up
BrowserAgents to handle the model-layer updates from overlay UI
interaction events.

Bug: 1030357
Change-Id: I4fa3f0c3afa6d0f747f0a7d3b9f5ee14dd43d4e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1978623
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728664}