Revert of Use SequenceChecker to allow Timer to run in SequencedWorkerPool (patchset #6 id:100001 of https://codereview.chromium.org/1433373003/ )

Reason for revert:
TSAN failures in the test. Won't have a chance to debug for a few days, so reverting for now.

Sample failure:

https://build.chromium.org/p/chromium.memory.fyi/builders/Linux%20TSan%20Tests/builds/14813

Original issue's description:
> 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
>
> Committed: https://crrev.com/ffa30fe7f5ec8b13c9e41d41d828e4fb8e67110b
> Cr-Commit-Position: refs/heads/master@{#365568}

TBR=gab@chromium.org,danakj@chromium.org,thakis@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=552633

Review URL: https://codereview.chromium.org/1532773002

Cr-Commit-Position: refs/heads/master@{#365683}
3 files changed