* 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
const DesktopMediaList::Source& source) {
    mate::Dictionary dict(isolate, v8::Object::New(isolate));
    content::DesktopMediaID id = source.id;
    dict.Set("name", base::UTF16ToUTF8(source.name));
    dict.Set("id", id.ToString());
    dict.Set(
        "thumbnail",
        atom::api::NativeImage::Create(isolate, gfx::Image(source.thumbnail)));
    return ConvertToV8(isolate, dict);
  }
};
}  
namespace atom {
namespace api {
DesktopCapturer::DesktopCapturer(v8::Isolate* isolate) {
  Init(isolate);
}
DesktopCapturer::~DesktopCapturer() {
}
void DesktopCapturer::StartHandling(bool capture_window,