Add amusbach@ to splitview OWNER file.

Bug: None
Change-Id: I0204923c6fc55f3318471041a68515c4fe391954
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2035859Reviewed-by: Mitsuru Oshima (slow in TOK) <oshima@chromium.org>
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737826}

ash/wm/splitview/OWNERS

+amusbach@chromium.org
 xdai@chromium.org
 
 # COMPONENT: UI>Shell>WindowManager>Splitscreen