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This method is not used outside of SequencedWorkerPool itself. Making it private
will prevent external users from using it directly, which will help the deprecation
of SequencedWorkerPool.

SequencedWorkerPool::IsRunningSequenceOnCurrentThread() is still used
by the RunsTasksOnCurrentThread() method of SequencedTaskRunners
returned by a SequencedWokrerPool.

BUG=622400

Review-Url: https://codereview.chromium.org/2341063002
Cr-Commit-Position: refs/heads/master@{#418984}