PostReview: Disallow modifying other people's drafts

Having permission to review on behalf of another user should not give
you permission to publish or delete their draft comments, as they were
not visible to you. And it's just not very nice.

Change-Id: I76247a055d43a6d68d16cba871f262a07063d9af

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/ReviewInput.java

@@ -49,8 +49,11 @@ public class ReviewInput {
   /**
    * How to process draft comments already in the database that were not also
    * described in this input request.
+   * <p>
+   * Defaults to DELETE, unless {@link #onBehalfOf} is set, in which case it
+   * defaults to KEEP and any other value is disallowed.
    */
-  public DraftHandling drafts = DraftHandling.DELETE;
+  public DraftHandling drafts;
 
   /** Who to send email notifications to after review is stored. */
   public NotifyHandling notify = NotifyHandling.ALL;