Fix the cancel button on the publish comments screen

It wasn't working because we didn't initialize the field,
so the click listener wasn't able to identify the sender
of the event, and thus didn't call the proper method.

Signed-off-by: Shawn O. Pearce <sop@google.com>

appjar/src/main/java/com/google/gerrit/client/changes/PublishCommentScreen.java

@@ -106,7 +106,7 @@ public class PublishCommentScreen extends AccountScreen implements
       send.addClickListener(this);
       buttonRow.add(send);
 
-      final Button cancel = new Button(Util.C.buttonPublishCommentsCancel());
+      cancel = new Button(Util.C.buttonPublishCommentsCancel());
       cancel.addClickListener(this);
       buttonRow.add(cancel);
     }