UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:57.0) Gecko/20100101 Firefox/57.0
Platform: 10032.86.0 (Official Build) stable-channel link

Steps to reproduce the problem:
1. create a new window, providing icon location in CreateWindowOptions (where this icon exists)
```
let options = {
  id: "new window id",
  resizable: false,
  outerBounds: {
    left: 0,
    top: 0,
    width: 150,
    height: 150,
  },
  showInShelf: true,
  icon: chrome.runtime.getURL('/images/newTrayIcon.png'),
};

chrome.app.window.create('window.html', options, callback);
```

2. 
a. when window is opened, the provided icon does not display
b. instead a generic icon is shown with a "badge" icon in the lower corner.  This "badge" is our main window icon.  However, our provided icon is not shown.

What is the expected behavior?
icon provided in CreateWindowOptions should not be ignored, and should display in the system tray

What went wrong?
icon is not displaying in the system tray

Did this work before? Yes OS version 62

Does this work in other browsers? Yes

Chrome version: 63.0.3239.140  Channel: stable
OS Version: 63.0.3239.140
Flash Version: 28.0.0.137

At first we thought it was a problem with chrome.runtime.getURL(), so we placed copies of the icon in multiple places in the app folder.  However, it did not work.