Fix undefined name 'Queue'.
Fix undefined name 'Queue'.

BUG=chromium-os:19386
TEST=pyflakes, manual run of cbuildbot

Change-Id: If3228755860334c88b769576199de6f98ec125fe
Reviewed-on: http://gerrit.chromium.org/gerrit/6321
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: Thieu Le <8cae94ed37a002427b56783feab9ab75ab7aa0b8@chromium.org>

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