ui/display/OWNERS: mark marcheu@ as legacy, add dcastagna@

This CL marks marcheu@ as legacy OWNER (a.k.a. OWNER of last resort)
and proposes dcastagna@ to step up.

Change-Id: I430e02174a02fae85a95b87e9ee0d1728520d98a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2432967Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811316}

ui/display/OWNERS

 afakhry@chromium.org
+dcastagna@chromium.org
 dnicoara@chromium.org
-marcheu@chromium.org
 oshima@chromium.org
 
+# Legacy OWNER(s)
+marcheu@chromium.org
+
 # COMPONENT: UI>Systems>Display