layout.conf: set empty masters

Some tools complain if masters doesn't exist.  Set it explicitly so that
they know this is a standalone repo.

BUG=chromium:205164
TEST=`pinspect` no longer complains

Change-Id: I651a10bcd94fa1bbfc9af8eb975dd964c7d7caa4
Reviewed-on: https://chromium-review.googlesource.com/180331
Reviewed-by: Chris Masone <cmasone@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>