Add github.com/bradfitz/gce to third_party.

Change-Id: Id5f504da159223ac6c695ea18b277ce1f3bc7960
googlecloudstorage: whren running on GCE, get access tokens automatically from the VM

Change-Id: Ibfcca0c52ff01a8f82bb0d3081119e0ac6e16ba9