How can I check if the browser support webkitdirectory?

I am using Google Chrome's folder upload feature in my project, described here: How do I use Google Chrome 11's Upload Folder feature in my own code?

I have a button that trigger an input field when be clicked. My question is How can I check if the browser support webkitdirectory or not? so I can hide my button or alert the user to use chrome for this service.

<button>Upload Folder</button>
<input type="file" name="file[]" multiple webkitdirectory>

<script>
  $("button").click(function(e) {
    /* TODO: Detect webkitdirectory support */
    if(webkitdirectory)
      $('input').trigger('click');
    else
      alert('Use Chrome!');
  });
</script>

Based on Modernizer and this answer I could use this function to detect if directory select is supported:

function testFileInputDirectory() {
    var elem = document.createElement('input'),
    dir = 'directory',
    domPrefixes = [ "", "moz", "o", "ms", "webkit" ],
    prefix;
    
    elem.type = 'file';
    
    for (prefix in domPrefixes) {
      if (domPrefixes[prefix] + dir in elem) {
        return true;
      }
    }
    return false;
}