* Electron version: 1.7.5
* Operating system: WIndows

### Expected behavior
On PC with 2 monitors, desktopCapturer.getSources return 2 screens. When change main screen, it show with title "Screen 1" secondary screen.

### Actual behavior
Method should correctly handle main screen switch and display with title "Screen 1" main screen.

### How to reproduce
[desktopCapturerScreenNumIssue.zip](https://github.com/electron/electron/files/1302236/desktopCapturerScreenNumIssue.zip)
atom/browser/api/atom_api_desktop_capturer.cc
bool capture_screen,
                                    const gfx::Size& thumbnail_size) {
  webrtc::DesktopCaptureOptions options =
      webrtc::DesktopCaptureOptions::CreateDefault();
#if defined(OS_WIN)
  
  
  
  
  
  
  options.set_disable_effects(false);
#endif
  std::unique_ptr<webrtc::DesktopCapturer> screen_capturer(
      capture_screen ? webrtc::DesktopCapturer::CreateScreenCapturer(options)
                     : nullptr);
  std::unique_ptr<webrtc::DesktopCapturer> window_capturer(
      capture_window ? webrtc::DesktopCapturer::CreateWindowCapturer(options)
                     : nullptr);
  media_list_.reset(new NativeDesktopMediaList(
      std::move(screen_capturer), std::move(window_capturer)));
  media_list_->SetThumbnailSize(thumbnail_size);
  media_list_->StartUpdating(this);
}
void DesktopCapturer::OnSourceAdded(int index) {
}
void DesktopCapturer::OnSourceRemoved(int index) {
}
void DesktopCapturer::OnSourceMoved(int old_index, int new_index) {