ozone/wayland: Extract subsurface from WaylandWindow

This CL does not bring any functional changes, but rather moves
the subsurface methods to WaylandSubsurface.

Test: ozone_unittests
Bug: 1028919
Change-Id: I96dbf21267a80022779cd7eebeb56c96a243128b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1960265
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Reviewed-by: Michael Spang <spang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#723758}
6 files changed