Device name: Nexus 6P

From "Settings > About Chrome"
Application version: 54.0.2840.61
Operating system: Android 6.0

URLs (if applicable): https://www.youtube.com

Steps to reproduce:
(1) Start any YouTube video
(2) Press Home button to return to phone Home screen, switching Chrome to background

Expected result:
The notification with media controls should be available.

Actual result:
The notification disappears as soon as application goes to background. So you couldn't listen to YouTube video sound while Chrome is in background mode.
It seems there is no such a problem on other sites, for example, https://www.dailymotion.com. After you start video there and switch Chrome to background mode, the notification with media controls appears, and you can easily play or pause the video without relaunching Chrome.