* 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
#include "atom/browser/api/atom_api_desktop_capturer.h"
#include <vector>
using base::PlatformThreadRef;
#include "atom/common/api/atom_api_native_image.h"
#include "atom/common/native_mate_converters/gfx_converter.h"
#include "base/strings/string_number_conversions.h"
#include "base/strings/utf_string_conversions.h"
#include "base/task_scheduler/post_task.h"
#include "base/threading/thread_task_runner_handle.h"
#include "chrome/browser/media/desktop_media_list.h"
#include "content/public/browser/browser_thread.h"
#include "content/public/browser/desktop_capture.h"
#include "native_mate/dictionary.h"
#include "third_party/webrtc/modules/desktop_capture/desktop_capture_options.h"
#include "third_party/webrtc/modules/desktop_capture/desktop_capturer.h"
#if defined(OS_WIN)
#include "third_party/webrtc/modules/desktop_capture/win/dxgi_duplicator_controller.h"
#include "ui/display/win/display_info.h"
#endif  
#include "atom/common/node_includes.h"
namespace mate {
template <>
struct Converter<atom::api::DesktopCapturer::Source> {
  static v8::Local<v8::Value> ToV8(