Revert "[blink] Remove old script streamer thread"

This reverts commit 96c748b0b0e61d207ef1d3bd70238b3443179b77.

Reason for revert: it's suspect in failures 
https://ci.chromium.org/p/chromium/builders/ci/Win7%20Tests%20%281%29/88931

Original change's description:
> [blink] Remove old script streamer thread
> 
> The script streamer thread has now been replaced with posting tasks to
> the scheduler, so remove the flag picking between the two options and
> the dedicated streaming thread itself.
> 
> Also, remove the streaming task re-posting now that the blocking/
> non-blocking worker pools are merged.
> 
> Bug: 874080
> Change-Id: Ib9dabb91deddcc616d4edfe5efb13d8e584c0691
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1503420
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Reviewed-by: Steven Holte <holte@chromium.org>
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
> Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
> Auto-Submit: Leszek Swirski <leszeks@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#638031}

TBR=kinuko@chromium.org,haraken@chromium.org,holte@chromium.org,mlippautz@chromium.org,leszeks@chromium.org

Change-Id: Ia67f3fcfbff3155f9da4b53f371115f3dfd0089c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 874080
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1505412
Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
Commit-Queue: Vadym Doroshenko <dvadym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638055}
18 files changed