Update OWNERS for ArcProcessService

This changes removes cmtm@ from ArcProcessService OWNERS, since he
no longers works on ARC, and adds wvk@ as a new owner.

BUG=b:156981234
TEST=none

Change-Id: I1082825aa594ef2a30a316d731d2b9b7fb43d2f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2208260
Auto-Submit: Willie Koomson <wvk@google.com>
Reviewed-by: Chris Morin <cmtm@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#769981}

chrome/browser/chromeos/arc/process/OWNERS

-cmtm@google.com
+wvk@google.com