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This is an addendum to r329262 which still didn't quite
get it right.

The assumption here is now that the IPCSupportInitializer
must outlive the IO thread anyway, so the IO loop observer
may (and should) call ForceShutdown immediately rather
than posting a task.

BUG=486004
R=jam@chromium.org

Review URL: https://codereview.chromium.org/1143433003

Cr-Commit-Position: refs/heads/master@{#329692}