Reland "parallel_emerge: Work around Portage library bug with usepkg"

This reverts commit 95f30c12f2f0012aa5bc45ee05b1cdeae9c7b9ea.

crrev.com/c/643566 fixed the original source of breakage, so this should
now be safe to put in again.

BUG=chromium:756240
TEST=Sent to chromiumos-sdk trybot

Change-Id: Icb9f5f7669e8e5514f698a175693203e06e3a8e9
Reviewed-on: https://chromium-review.googlesource.com/644617
Commit-Ready: Benjamin Gordon <bmgordon@chromium.org>
Tested-by: Benjamin Gordon <bmgordon@chromium.org>
Reviewed-by: Jason Clinton <jclinton@chromium.org>
1 file changed