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Using a 4x4 sized opaque background will not regress performance in
underlay cases because the root surface is still opaque. The background
will be stretched out by wayland viewporter extension.

This is 6/? CL for overlay forwarding using wl_subsurface.

Change-Id: I59b0d4cdbfbb6727d578e286c9d19cf69a9bff32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2352523
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@{#810527}