Add amistry, dats, sammc, and slangley to file manager owners.

And remove yamaguchi.

BUG=None

Change-Id: Icac3b856995fb0b60b0b309825852715de99059b
Reviewed-on: https://chromium-review.googlesource.com/1235357Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Stuart Langley <slangley@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Anand Mistry <amistry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592692}

ui/file_manager/OWNERS

+amistry@chromium.org
+dats@chromium.org
 fukino@chromium.org
 joelhockey@chromium.org
 lucmult@chromium.org
 noel@chromium.org
-yamaguchi@chromium.org
+sammc@chromium.org
+slangley@chromium.org