background.js

//from: https://stackoverflow.com/questions/7699615/cross-domain-xmlhttprequest-using-background-pages

chrome.runtime.onMessage.addListener(function(request, sender, callback) {
  if (request.action == "xhttp") {
      var xhttp = new XMLHttpRequest();
      var method = request.method ? request.method.toUpperCase() : 'GET';

      xhttp.onload = function() {
          callback(xhttp.responseText);
      };
      xhttp.onerror = function() {
          console.error("request failed!");
          callback();
      };
      xhttp.open(method, request.url, true);
      if (method == 'POST') {
          xhttp.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
      }
      xhttp.send(request.data);
      return true; // prevents the callback from being called too early on return
  }
});