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Years ago DebugLogWriter used runners from SequencedWorkerPool to gather
and archive logs, thus requiring sequence_token_name argument to identify pool.

In b116e3b7 this was deprecated, work
migrated to TaskScheduler. But sequence_token_name argument persisted
and was passed into different methods without any real usage.

Also migrate from legacy Callback to OnceCallback.

Change-Id: If2b650fd6d7221be75002db28dd4afb46c14d6a5
Reviewed-on: https://chromium-review.googlesource.com/c/1480520Reviewed-by: Rahul Chaturvedi <rkc@chromium.org>
Reviewed-by: Denis Kuznetsov <antrim@chromium.org>
Commit-Queue: Oleg Davydov <burunduk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635935}