Revert "Store the current SequenceManagerImpl instance in TLS"

This reverts commit 426952a6d28d89caed29b48ef4455a79f1c0d964.

Reason for revert: Causes builder Mac10.12 Tests to fail. See bug 943537

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}

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

Change-Id: I64971722513c63385aea97d61b5c4ddba267faa7
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 891670, 943537
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1530841
Reviewed-by: Alexander Hendrich <hendrich@chromium.org>
Commit-Queue: Alexander Hendrich <hendrich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641850}
4 files changed