Deleting another user's vote on a change is implemented as inserting a 0 vote for that label. A subsequent 0 vote by the user is interpreted by PostReview as a request to delete that vote. A bug in validatePostSubmitLabels was treating all delete requests as reducing votes, which is not the case. Bug: Issue 4918 Change-Id: Id8091558edeb0f77542789f3a53841972a2ac1bc