cros: Update wallpaper owner file

Bug: NONE
Change-Id: Ic15fc24e7bf75d259084566cefdefe7b3102a20f
Reviewed-on: https://chromium-review.googlesource.com/1152694Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578706}

ash/wallpaper/OWNERS

 achuith@chromium.org
-bshe@chromium.org
 wzang@chromium.org
 xdai@chromium.org
 

chrome/browser/resources/chromeos/wallpaper_manager/OWNERS

-bshe@chromium.org
 wzang@chromium.org
 xdai@chromium.org