Allow downloading prebuilts without a boto file.
Allow downloading prebuilts without a boto file.

BUG=chromium:357888
TEST=emerge-x86-generic -g chromeos without a boto file

Change-Id: Id7671d5acf704ce3e7badb0e532ed15c3c29e929
Reviewed-on: https://chromium-review.googlesource.com/197063
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

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