Webview sample: Inject script into main world (not isolated world)

This sample shows how to add a script tag to the DOM, effectively injecting script into the main world rather than the isolated world where normal webview.executeScript() code runs.
First time I ran the app I didn't notice right away that there were two windows. It'd be nice if we could have something like:

```javascript
window.chrome.app.window.create('correct_injection.html', {'id': 'goodWindowID'},
    function(appGoodWindow) {
  var offset = 20;
  var badWindowOuterBounds = {
    'left': appGoodWindow.outerBounds.left + offset,
    'top': appGoodWindow.outerBounds.top + offset
  };
  window.chrome.app.window.create('incorrect_injection.html', {'id': 'badWindowID', 'outerBounds': badWindowOuterBounds});
});