fieldsEqual = function(value1, value2) {
        'use strict';
        return isComposite ? value1.equals(value2) : value1 == value2;
      }
Compares this message with another one ignoring the unknown fields.
@param {?} other The other message.
@return {boolean} Whether they are equal. Returns false if the `other`
    argument is a different type of message or not a message.