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This CL adds functionality to OverlayPresenter to drive the
presentation of overlay UI.

It also updates the presenter to observe its WebStateList
and OverlayRequestQueueImpls for the lifetime of the object,
rather than only when a UI delegate is provided.
ShowNextOverlay() no-ops without a UI delegate, and is already
called in scenarios where an overlay may not be shown.

Bug: 941745
Change-Id: Ib8fe7b49bfba3d8624c5acf40f7e0f512d6cd0ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1586776
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#656401}