Google Chrome	55.0.2883.42 (Official Build) beta (64-bit)
Revision	0
Platform	8872.44.0 (Official Build) beta-channel samus

What steps will reproduce the problem?
(1) Attach a USB3 device that supports MTP (Pixel phone or tablet)
(2) Look at files.app

What is the expected result?
You can browse MTP like you can with USB2 devices

What happens instead?
No device visible.

Please use labels and text to provide additional information.
Device enumerates properly according to lsusb and dmesg, so I guess it's a udev or libmtp issue.