* 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
#include "atom/browser/api/atom_api_desktop_capturer.h"
using base::PlatformThreadRef;
#include "atom/common/api/atom_api_native_image.h"
#include "atom/common/native_mate_converters/gfx_converter.h"
#include "base/strings/utf_string_conversions.h"
#include "chrome/browser/media/desktop_media_list.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"
#include "atom/common/node_includes.h"
namespace mate {
template<>
struct Converter<DesktopMediaList::Source> {
  static v8::Local<v8::Value> ToV8(v8::Isolate* isolate,