Suppress sequence checks in SyncFileSystem on the shutdown phase

Suppress sequence checks in SyncFileSystem on the shutdown phase

SyncFileSystem stuff hits DCHECK or NOTREACHED on shutdown phase. That
is due to failing PostTask() SequencedWorkerPool.
This CL relaxes several assertions in the case.

* Disable TaskToken tracking if SequencedWorkerPool is being shut down.
* Leak SyncWorker if SequencedWorkerPool is being shut down, instead of
  deleting it in the other thread.

BUG=453226
Committed: https://crrev.com/3ad68aba0b036b9eb2c67738c83c8dfed1fe6fe5
Cr-Commit-Position: refs/heads/master@{#314503}

[tzik, 2015-02-03 06:52:00]

tzik@chromium.org changed reviewers:
+ nhiroki@chromium.org

[tzik, 2015-02-03 06:52:53]

PTAL

[nhiroki, 2015-02-04 03:37:28]

lgtm

[tzik, 2015-02-04 03:48:58]

The CQ bit was checked by tzik@chromium.org

[commit-bot: I haz the power, 2015-02-04 03:50:08]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/881203003/20001

[commit-bot: I haz the power, 2015-02-04 04:17:06]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-02-04 04:18:11]

Patchset 2 (id:??) landed as
https://crrev.com/3ad68aba0b036b9eb2c67738c83c8dfed1fe6fe5
Cr-Commit-Position: refs/heads/master@{#314503}