Revert "Run archive_build in foreground again."
Revert "Run archive_build in foreground again."

Now that the flakiness with the test harness has been fixed, we can run
archive_build in the background again.

This reverts commit b1473836cd03c8bf2df5ec3f3fbe759a54bc76b8.
BUG=chromium-os:12220
TEST=Run example buildbot run and unit tests.

Change-Id: I52e3c3f369da83c149201a1de8f1e99aa7a3cf02
Reviewed-on: http://gerrit.chromium.org/gerrit/4818
Reviewed-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,zhang0137/chromite,chadversary/chromiumos.chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,coreos/chromite,zhang0137/chromite