Update OWNERS for window-service

Adds xiyuan and mukai

BUG=none
TEST=none

Change-Id: I1bdbc6251e3e3967dd05bda84b7c498527e0ff95
Reviewed-on: https://chromium-review.googlesource.com/c/1279233Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599400}

services/ws/OWNERS

 # COMPONENT: Internals>Services>WindowService
 
 msw@chromium.org
+mukai@chromium.org
 sadrul@chromium.org
 sky@chromium.org
+xiyuan@chromium.org
 
 per-file *_type_converter*.*=set noparent
 per-file *_type_converter*.*=file://ipc/SECURITY_OWNERS