Enable config'd postMessage in paymentSuccess/Failure (bug 1101004)

r? @kumar303 
if you really think we need this flag then the ``window.opener`` check should be separate, like:
````
if (settings.enableWebPayments === true) {
  if (!window.opener) {
    console.error('refusing to communicate via postMessage since there is no window.opener');
  } else {
    // do postMessage...
  }
} else {
  // B2G stuff...
}
````