Use SequenceChecker to allow Timer to run in SequencedWorkerPool

Timer was explicitly ensuring single-threaded usage by tracking the
thread used for scheduling tasks. Generalize that to ensure sequenced
task runner affinity via SequenceChecker, so it can be used by
SequencedWorkerPool threads.

BUG=552633

Review URL: https://codereview.chromium.org/1433373003

Cr-Commit-Position: refs/heads/master@{#365568}
3 files changed