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This patch does not bring functionality changes, but
just wraps the wl_shm to WaylandShm for more convinient usage.

This is also a prerequisite to unite the
WaylandShmBufferManager and the WaylandBufferManager.

Bug: 963853
Change-Id: I5ce47dc981caccf3df7f0092cb2095712a63443e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617501Reviewed-by: Michael Spang <spang@chromium.org>
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#661174}