* Electron Version: 3.0.0-beta.2
* Operating System (Platform and Version): Current Windows 10, 64bit
* Last known working Electron version: 2.0.4

**Expected Behavior**
Full screen and all opened windows to be returned.

**Actual behavior**
Only full screen and a seemingly identical item are returned. Independent of the number of opened windows. 

**To Reproduce**
Call
```
desktopCapturer.getSources({
    types: ['window', 'screen'],
    thumbnailSize: {
          width: 250,
          height: 250
    }
}, function (err, sources) {

});
```
The `sources` contains only two items, the first one, which is the entire desktop screen is valid, the other one is not.


atom/browser/api/atom_api_desktop_capturer.cc
const std::vector<atom::api::DesktopCapturer::Source>& sources,
    atom::api::DesktopCapturer* cap) {
  cap->Emit("finished", sources);
}
void StartHandlingTask(bool capture_window,
                       bool capture_screen,
                       const gfx::Size& thumbnail_size,
                       atom::api::DesktopCapturer* cap) {
#if defined(OS_WIN)
  cap->using_directx_capturer_ =
      content::desktop_capture::CreateDesktopCaptureOptions()
          .allow_directx_capturer();
#endif  
  std::unique_ptr<webrtc::DesktopCapturer> screen_capturer(
      capture_screen ? content::desktop_capture::CreateScreenCapturer()
                     : nullptr);
  std::unique_ptr<webrtc::DesktopCapturer> window_capturer(
      capture_window ? content::desktop_capture::CreateScreenCapturer()
                     : nullptr);
  cap->media_list_.reset(new NativeDesktopMediaList(
      std::move(screen_capturer), std::move(window_capturer)));
  cap->media_list_->SetThumbnailSize(thumbnail_size);
  cap->media_list_->StartUpdating(cap);
}
void OnRefreshFinishedTask(atom::api::DesktopCapturer* cap) {