CROS_OWNERS: add more build peeps

Change-Id: I15763fa3b1a42904eb4434f4ca72b6ad3bd0edb8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2812198
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Stephane Belmon <sbelmon@google.com>

CROS_OWNERS

 # Chrome OS developers looking over Chrome OS related files.
 cjmcdonald@chromium.org
 lamontjones@chromium.org
-mmortensen@google.com
 nednguyen@chromium.org
 saklein@chromium.org
+sbelmon@google.com
+sfrolov@google.com
 vapier@chromium.org