title: Chrome USB API - How do I get the ConnectionHandle
body: <p>I've been looking at USB Api to connect to a USB relay.</p>
  
  <p>Using chrome.usb.findDevices I can open the device but then I need to send some data to it. I'm thinking chrome.usb.controlTransfer is the way to do that. I have no idea how to get the ConnectionHandle which I need for the next step.</p>
  
  <p>My findDevices code is below. </p>
  
  <p>Any help would be gratefully received. I'm properly confused.</p>
  
  <pre><code>var usbConnection;
  
  var onOpenCallback = function(connection) {
    if (connection) {
      usbConnection = connection;
      document.getElementById("found").innerHTML ="Device opened.";
  
    } else {
      document.getElementById("found").innerHTML ="Device failed to open.";
    }
  }; 
  
  chrome.usb.findDevices(DEVICE_INFO, onOpenCallback);
  </code></pre>