Add xdai@, mukai@ to wm owners

Bug: None
Change-Id: Icc08efd9ec16d63e4ba841778a90ce10d662b6bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1733583
Auto-Submit: Mitsuru Oshima (OOO 8/5,6) <oshima@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684002}

ash/wm/OWNERS

 flackr@chromium.org
+mukai@chromium.org
+xdai@chromium.org
 
 # COMPONENT: UI>Shell>WindowManager

ash/wm/overview/OWNERS

 sammiequon@chromium.org
-xdai@chromium.org
 
 # COMPONENT: UI>Shell>OverviewMode