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There is no need to remove the IO TaskRunner reference.
The global io thread task_runner is never deleted.
If io TaskRunner is set, just ignore future calls.

BUG=842245

Change-Id: Ifefad6442d8efde238379218e9f11a9f3d99a75a
Reviewed-on: https://chromium-review.googlesource.com/1134197Reviewed-by: Min Qin <qinmin@chromium.org>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574402}