Fix AssertionError on bots that don't upload prebuilts.
Fix AssertionError on bots that don't upload prebuilts.

BUG=chromium-os:17243
TEST=Unit tests. Ran sample run of tegra2_seaboard-toolchain bot.

Change-Id: Ie6c8885c25e2a7f9022796882840459656839387
Reviewed-on: http://gerrit.chromium.org/gerrit/3558
Reviewed-by: Raymes Khoury <348ad72dc039a127ca70ef38e5de4eebe052b884@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

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