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: https://nativeclient-sdk.googlecode.com/svn/trunk/src@1574 050acbb0-2703-11df-ab0a-9f3f633ae91d