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|child_binding_| was introduced by r461826 to be bound on the IO
thread. This CL ensures that it's also unbound on the same thread
during shutdown; otherwise there's a race and it might get
incorrectly unbound on the main thread, leading to a DCHECK in
developer builds.

BUG=716416
TBR=ben@chromium.org

Change-Id: I194d1446753d3fb976b21bb191e7a407e7f8c6e7
Reviewed-on: https://chromium-review.googlesource.com/491886Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#468405}