GitLab

Rename OnOptInEnabled to OnArcOptInChanged (to make it
clearer that it's also called on opt-out), and add "Arc" to
other method names as well to make their meaning clearer
when implemented by non-ARC-specific classes.

BUG=633243

Review-Url: https://codereview.chromium.org/2577103002
Cr-Commit-Position: refs/heads/master@{#438920}