A repro code for Tray#displayBalloon's icon

This is a repro code about Tray#displayBalloon's icon parameter does not work.

Environments

• Electron version: 1.7.9
• Operating system: Windows 7 Pro SP1 (64 bits)

Steps

1. git clone https://github.com/mysticatea/electron-tray-test.git
2. cd electron-tray-test
3. npm install
4. npm run test:none ... It works if the icon parameter is nothing.
5. npm run test:string ... It does not work if the icon parameter is a string. It showed the tray icon, but does not show the balloon.
6. npm run test:image ... It does not work if the icon parameter is a NativeImage instance. It showed the tray icon, but does not show the balloon.