Simple load ajax works in Firefox peron does not work in Chrome

I'm trying an example in Ajax that apparently only works in Firefox. In Chrome it does not work. Does anyone know why? Thanks

index.html

  <div class="contenedor">
        <p>AJAX</p>          
        <button id="cargar" value="cargar">Cargar</button>
        <div id="contenido"></div>
  </div>
  <script src="js/06.js"></script>

06.js

var btnCargar = document.getElementById('cargar');

function cargarContenido() {
  // se crea
  var xhr = new XMLHttpRequest();

  // se abre
   xhr.open("GET", "servidor.php", true);

   // revisar que cambie
   xhr.onreadystatechange = function() {
      console.log(xhr.readyState);

      if(xhr.readyState == 4 && xhr.status == 200) {
          //console.log("Se cargo correctamente");
          var json = JSON.parse(xhr.responseText);

          //console.log(json);
          var contenido = document.getElementById('contenido');
          contenido.innerHTML = json.frontend;
      } 
   };
   xhr.send(); 
}

btnCargar.addEventListener('click', cargarContenido );

server.php

$tecnologias = array(
    'frontend' => 'html5, css3, javascript',
    'backend' => 'php, mysql, apache',
    'fullstack' => 'html, css3, javascript, php, mysql, apache'
);
echo json_encode($tecnologias);