Google Chrome	71.0.3558.0 (Official Build) dev (64-bit)
Revision	09438e58ad92274e6fa5f7a76429836ceeba5fa5-refs/branch-heads/3558@{#1}
Platform	11097.0.0 (Official Build) dev-channel eve
Firmware Version	Google_Eve.9584.171.0

What steps will reproduce the problem?
1. Use a device in tablet mode.
2. Open a new tab and navigate to a url.
3. Press shelf back button.

What is the expected result?
The shelf back button should be disabled or grayed out like Chrome's back button.


What happens instead of that?
The shelf back button does not change in any way.