UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.106 Safari/537.36

Steps to reproduce the problem:
window.onbeforeunload = () => 'Are you sure want to leave this page';
// or
window.addEventListener('beforeunload', e => {
  e.returnValue = 'Are you sure want to leave this page';
})

What is the expected behavior?
The confirm navigation popup show "Are you sure want to leave this page".

What went wrong?
The confirm navigation popup show the default message "Changes you made may not be saved".

Did this work before? Yes 

Chrome version: 51.0.2704.106  Channel: stable
OS Version: 10.0
Flash Version: Shockwave Flash 22.0 r0