Make the tag field public.

Change-Id: Ibfda5bb1ac3150ea90d82a152730be76a2f8bf71

java/src/main/java/com/google/protobuf/nano/Extension.java

@@ -138,7 +138,7 @@ public class Extension<M extends ExtendableMessageNano<M>, T> {
     /**
      * Tag number of this extension.
      */
-    protected final int tag;
+    public final int tag;
 
     /**
      * Whether this extension is repeated.