UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.113 Safari/537.36

Steps to reproduce the problem:
1. Create a HTML page with @page { size: A4; }
2. Convert to PDF with headless Chromium
3. Check dimensions of PDF

What is the expected behavior?
The dimensions of the generated PDF should be A4 (210x297mm).

What went wrong?
The dimensions of the generated PDF is US Letter (8.5in x 11in).

Did this work before? N/A 

Does this work in other browsers? Yes

Chrome version: 60.0.3112.78  Channel: stable
OS Version: Debian Stretch
Flash Version: 

When I open the page as HTML in my Chrome browser (non-headless), press the Print icon and then choose "Save as PDF", it correctly adopts the @page size I set (in my case A4). The bug only occurs in headless mode.