Remove implicit conversion from scoped_refptr to T* in UserSessionManager

This wasn't caught by the trybots or CQ since this is only compiled in
official builds.

BUG=110610
TBR=davemoore@chromium.org

Review URL: https://codereview.chromium.org/574793004

Cr-Commit-Position: refs/heads/master@{#295237}
1 file changed