Fixes possible use after free in SessionService

      SessionService::GetLastSession used a base::Unretained but there was
no guarantee that the SessionService would be valid by the time the
callback was processed.

BUG=399655
TEST=covered by test now
R=marja@chromium.org

Review URL: https://codereview.chromium.org/500143002

Cr-Commit-Position: refs/heads/master@{#291985}