UserAgent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.133 Safari/537.36

Steps to reproduce the problem:
1. Start chromium with: chromium-browser --user-agent="foo" --headless
2. Check the "User-Agent" it sends
3. It is: "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Ubuntu HeadlessChrome HeadlessChrome Safari/537.36"

What is the expected behavior?
It should be "foo"

What went wrong?
It seems to ignore the "--user-agent" flag. If I do not use "headless" it works perfectly fine.

Did this work before? N/A 

Does this work in other browsers? Yes

Chrome version: 57.0.2987.98  Channel: stable
OS Version: 16.04
Flash Version: