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WaylandSubsurface wraps a WaylandSurface and wl_subsurface, it can be
configured by |parent_| to forward overlays scheduled on WaylandWindow.

Proposal doc: https://docs.google.com/document/d/1qdioeaZmfv8df0qbVTVoVQ8qSaHBZ0T3cjF4eEpARi8/edit?usp=sharing

This is 2/? CL for overlay forwarding using wl_subsurface.

Bug: 1063865
Change-Id: I45397fa00b5deb958d4e9c519626a56cecf88f68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2288073
Commit-Queue: Kramer Ge <fangzhoug@chromium.org>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Maksim Sisov (GMT+3) <msisov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#793372}