Revert "Feature to run Blink main thread using SequenceManager"

This reverts commit 66fbaed95264740edc1ba7c51ab7b5401a31aeb2.

Reason for revert: Revert required to allow https://chromium-review.googlesource.com/c/chromium/src/+/1354919 to be reverted cleanly for issue 910645.

Original change's description:
> Feature to run Blink main thread using SequenceManager
>
> This will enable us to start trialing the MessageLoop replacement by
> SequenceManager.
>
> We only do this in the Blink main thread for now. There is some work
> needed around SequenceManager ownership to be done before this can be
> enabled in worker threads.
>
>
> Bug: 891670
> Change-Id: I1e59784ff35c87883a66d9b315435bf47cd0f809
> Reviewed-on: https://chromium-review.googlesource.com/c/1349316
> Reviewed-by: Alex Clarke <alexclarke@chromium.org>
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Reviewed-by: Alexander Timin <altimin@chromium.org>
> Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
> Commit-Queue: Carlos Caballero <carlscab@google.com>
> Cr-Commit-Position: refs/heads/master@{#612185}

TBR=dcheng@chromium.org,skyostil@chromium.org,alexclarke@chromium.org,altimin@chromium.org,carlscab@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 910645, 891670
Change-Id: I3529309984cd61dfef8a10e49129d364efabcf1f
Reviewed-on: https://chromium-review.googlesource.com/c/1357016
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612737}
7 files changed