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Observers who had started UI because of overlay presentation previously
would have had to store state to track whether the UI has been started
so that it could be stopped when the corresponding DidHideOverlay()
signal was received.  This CL updates DidHideOverlay() to include the
request corresponding with the overlay that was stopped so observers
could detect whether UI needs to be updated based off of the request
instead of storing this state.

Implementing this callback required updating OverlayRequestQueueImpl
to return unique pointers of popped requests so that the request
lifetime can be extended beyond the observer callback.

Bug: 941745
Change-Id: I256fa21766148ea4b8856c86e3d3e99517b48be4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1659232
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Auto-Submit: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671466}