MD Settings: fix session restore when the MaterialDesignSettings feature changes across restarts

Note: this has the small side effect of:

chrome://help
-> enable md-settings, restart
chrome://settings/help
-> disable md-settings, restart
chrome://settings

R=thestig@chromium.org
BUG=700350

Review-Url: https://codereview.chromium.org/2770913003
Cr-Commit-Position: refs/heads/master@{#459047}
1 file changed