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This change removes hard requirement on a wl_seat and allows
to run Ozone/Wayland without that. This is required by 1)
tests that will be run with headless weston 2) systems that
run without any perihperial peripheral devices.

TBR=spang@chromium.org

Bug: 1029241
Change-Id: I9970052b3d6d593591eb93738d0b83218abd1353
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088852Reviewed-by: Maksim Sisov <msisov@igalia.com>
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#747153}