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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/1349316Reviewed-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}