childHandler = function(child) {
          'use strict';
          if (child) {
            var doc = goog.dom.getOwnerDocument(contentElem);
            contentElem.appendChild(
                typeof child === 'string' ? doc.createTextNode(child) : child);
          }
        }
Takes a control's root element, and sets its content to the given text
caption or DOM structure.  The default implementation replaces the children
of the given element.  Renderers that create more complex DOM structures
must override this method accordingly.
@param {Element} element The control's root element.
@param {goog.ui.ControlContent} content Text caption or DOM structure to be
    set as the control's content. The DOM nodes will not be cloned, they
    will only moved under the content element of the control.