Migrate assist_ranker to BindOnce

Bug: 714018
Change-Id: Ied2384bf97e7f93449874d768b047c37e0474548
Reviewed-on: https://chromium-review.googlesource.com/1131570
Reviewed-by: Roger McFarlane <rogerm@chromium.org>
Commit-Queue: Mark Pilgrim <pilgrim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574254}
3 files changed