If you are a Googler, you can use gs://chromeos-throw-away-bucket for testing. Keep in mind that the contents can be read or deleted by anyone at Google.
We also have another similar bucket named gs://chromeos-shared-team-bucket . Probably using gs://chromeos-throw-away-bucket makes more sense since it is clear that the contents there can be deleted by anyone at Google.
If you are not a Googler, you should create your own bucket in Google Storage.
emerge -g --usepkgonly (Note: The --getbinpkg only flag for emerge does not actually work)
Emerge will tell you what USE flags are used when the -pv flags are provided e.g.
emerge-$BOARD chromeos-chrome -pv
FAQ items covering general development questions