switch from chromeos-base/chromeos to virtual/target-os

This is mechanical -- virtual/target-os depends on chromeos,
so there shouldn't be any real difference in the build.

BUG=chromium:329920
TEST=`cbuildbot {arm,amd64,x86}-generic-full` pass
TEST=`cbuildbot beaglebone` passes
CQ-DEPEND=CL:192487

Change-Id: Id83c6f8b585faeef3548430a15e9d32872a8d780
Reviewed-on: https://chromium-review.googlesource.com/192991
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
3 files changed