Revert of Remove usage of SequencedWorkerPool::GetNamedSequenceToken from content::CreateFileSystemContext. (patchset #1 id:1 of https://codereview.chromium.org/2883403002/ )

Reason for revert:
Seems to cause data races, since different FileSystemContexts are no longer sharing the same sequence.

Original issue's description:
> Remove usage of SequencedWorkerPool::GetNamedSequenceToken from content::CreateFileSystemContext.
>
> SequencedWorkerPool is being deprecated in favor of TaskScheduler.
>
> BUG=667892
>
> Review-Url: https://codereview.chromium.org/2883403002
> Cr-Commit-Position: refs/heads/master@{#474638}
> Committed: https://chromium.googlesource.com/chromium/src/+/bb731a6137ac921a8aa465613acab752b16f4666

TBR=tzik@chromium.org,fdoray@chromium.org
BUG=667892
NOPRESUBMIT=true

Review-Url: https://codereview.chromium.org/2969293003
Cr-Commit-Position: refs/heads/master@{#484638}
1 file changed