static getElementByClassName(className, parent) {
    const elements = parent.getElementsByClassName(className);
    goog.asserts.assert(elements.length == 1,
        'Should only be one element with class name ' + className);

    return shaka.util.Dom.asHTMLElement(elements[0]);
  }
Returns the element with a given class name.
Assumes the class name to be unique for a given parent.

@param {string} className
@param {!HTMLElement} parent
@return {!HTMLElement}