Added alias getFieldProto3 as used by older generated code.

Un-breaks users who have old generated code and upgrade
to the 3.1.0 release.

js/message.js

@@ -754,6 +754,18 @@ jspb.Message.getFieldWithDefault = function(msg, fieldNumber, defaultValue) {
 };
 
 
+/**
+ * Alias for getFieldWithDefault used by older generated code.
+ * @template T
+ * @param {!jspb.Message} msg A jspb proto.
+ * @param {number} fieldNumber The field number.
+ * @param {T} defaultValue The default value.
+ * @return {T} The field's value.
+ * @protected
+ */
+jspb.Message.getFieldProto3 = jspb.Message.getFieldWithDefault;
+
+
 /**
  * Gets the value of a map field, lazily creating the map container if
  * necessary.