Only skip uploading of addin if the file already exists on google storage.
Only skip uploading of addin if the file already exists on google storage.

The means the periodic builder can upload a revision if the
main builder happened flake and not upload first.

R=binji@chromium.org

Review URL: https://codereview.chromium.org/288983005

git-svn-id: e22621a84199d42b34715cdc089545aeedbbf23f@1574 050acbb0-2703-11df-ab0a-9f3f633ae91d

sbc100/nativeclient-sdk,sbc100/nativeclient-sdk,sbc100/nativeclient-sdk,sbc100/nativeclient-sdk,sbc100/nativeclient-sdk,sbc100/nativeclient-sdk,sbc100/nativeclient-sdk