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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.

Original change https://chromium-review.googlesource.com/c/1349316

TBR=dcheng@chromium.org

Bug: 891670
Change-Id: I68a59ec2d780106bb5fd76cee5976e236f9316f6
Reviewed-on: https://chromium-review.googlesource.com/c/1358452
Commit-Queue: Carlos Caballero <carlscab@google.com>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613132}