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Set up an overlay manager that allows an exo::ShellSurface to become an
overlay on top of a different window.

The initial use case is for a Android Play Billing popup to overlay a
Trusted Web App, when the app invokes a Payments API.

The shell surface must use the revised
aura_shell_surface::set_client_surface_str_id request to set a valid
overlay token.

The code to have the payment component hook into the overlay manager is
in a followup CL.

Bug: b:172592701
Test: Test Payment app shows the Play Store overlay

Change-Id: I7929b0a28ac4f4dd6e069632ada3419062f1b794
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519929
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Auto-Submit: Lloyd Pique <lpique@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826508}