* 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
Init(isolate);
  capture_thread_ = base::CreateSequencedTaskRunnerWithTraits(
      {base::WithBaseSyncPrimitives(), base::MayBlock(),
       base::TaskPriority::USER_VISIBLE});
}
DesktopCapturer::~DesktopCapturer() {}
void DesktopCapturer::StartHandling(bool capture_window,
                                    bool capture_screen,
                                    const gfx::Size& thumbnail_size) {
  capture_thread_->PostTask(
      FROM_HERE, base::BindOnce(StartHandlingTask, capture_window,
                                capture_screen, thumbnail_size, this));
}
void DesktopCapturer::OnSourceAdded(int index) {}
void DesktopCapturer::OnSourceRemoved(int index) {}
void DesktopCapturer::OnSourceMoved(int old_index, int new_index) {}
void DesktopCapturer::OnSourceNameChanged(int index) {}