Migrate captive_portal to BindOnce

Bug: 714018
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ia2bf1a8fa963f7540272a0d28961bfc961f14dd0
Reviewed-on: https://chromium-review.googlesource.com/1131994
Reviewed-by: Adrienne Porter Felt <felt@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Mark Pilgrim <pilgrim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574620}
6 files changed