Reland "Store the current SequenceManagerImpl instance in TLS"

This is a reland of 426952a6d28d89caed29b48ef4455a79f1c0d964

Seems that the Mac failure is a flake. I can see similar recent errors
on the builder. Trybot seems happy.

Original change's description:
> Store the current SequenceManagerImpl instance in TLS
>
> Now that the backend for MessageLoop is always a SequenceManager it no
> longer makes sense to have the TLS logic in MessageLoopCurrent.
>
> Eventually MessageLoopBase will disappear completely and
> MessageLoopCurrent will use the API exposed by SequenceManager. This
> patch is a step in that direction.
>
> Bug: 891670
> Change-Id: I24a76ded54fcf4b95bc8192ce0fe413aaeee162d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1497074
> Commit-Queue: Carlos Caballero <carlscab@google.com>
> Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
> Reviewed-by: Gabriel Charette <gab@chromium.org>
> Reviewed-by: Alexander Timin <altimin@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#641834}

Bug: 891670
Bug: 943537
Change-Id: I0799a575d2e16411311dc281c17ebd98284fc12c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1530771
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#642458}
4 files changed