Reland "Reland "[blink] Remove old script streamer thread""

This is a reland of 9d9114047e6e54a39cfac51c6e2f39877227bb50

Disabled the flaky test for now, the shipping config doesn't use the script streamer
thread so we want to keep it tested by the other tests. I'll fix the flake in a
followup.

Original change's description:
> Reland "[blink] Remove old script streamer thread"
>
> This is a reland of 96c748b0b0e61d207ef1d3bd70238b3443179b77
>
> Robustified the flaky test by using TestingPlatformSupportWithMockScheduler
> to run the scheduler until exhaustion (fun fact, seems that the live version
> of the script streaming wasn't being tested by this unit test :/ )
>
> 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=mlippautz@chromium.org,holte@chromium.org,haraken@chromium.org,kinuko@chromium.org
>
> Bug: 874080
> Change-Id: I5be59d64fc2d6074094ce57cf884bcd0bdffd3ce
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1507874
> Reviewed-by: Leszek Swirski <leszeks@chromium.org>
> Reviewed-by: Alexander Timin <altimin@chromium.org>
> Commit-Queue: Leszek Swirski <leszeks@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#638553}

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

Bug: 874080, 866868, 939054
Change-Id: I7fc75fcc9be50d6a8373a7efe5c296f8210c994b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529289
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642455}
18 files changed