GitLab

MDC has a method to use the old style, but it's in a private component
and the wind is blowing towards removing it at some point, so this CL
just opt-s out of MDC driving the back button in Settings.

I still filed a request at MDC to have the method public, but it might
never come, or they might remove the old style before that.

BUG=none
R=sdefresne@chromium.org

Review-Url: https://codereview.chromium.org/2589193002
Cr-Commit-Position: refs/heads/master@{#439776}