Fix gsutil -z option.
Fix gsutil -z option.

TBR=ppi@chromium.org

Review URL: https://codereview.chromium.org/1243223002 .

afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo