Reland "[blink] Remove old script streamer thread"

This reverts commit 50b4e18615b57365d245f3f9993faca743a398eb.

Reason for revert: It was incorrect suspicion, bot became green w/o this revert

Original change's description:
> 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}

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

Change-Id: Ic4eb793dacdaa7565e92ba1b908162f5d3e02e15
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 874080
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1505552
Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
Commit-Queue: Vadym Doroshenko <dvadym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638059}
18 files changed