This reverts commit 7dcc8eff.

Reason for revert: Some flakes still (see comments) and breaks predictable testing:
https://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20predictable/builds/11452

Original change's description:
> Revert "Revert "Introducing an event loop mechanism for d8.""
> 
> This reverts commit f7c25da6.
> 
> Reason for revert: Fixed
> 
> Original change's description:
> > Revert "Introducing an event loop mechanism for d8."
> > 
> > This reverts commit de964dbe.
> > 
> > Reason for revert:
> > https://build.chromium.org/p/client.v8/builders/V8%20Linux/builds/17958
> > 
> > Original change's description:
> > > Introducing an event loop mechanism for d8.
> > > 
> > > This mechanism ensures APIs like wasm async complete their work, 
> > > without requiring use of natives (%APIs).
> > > 
> > > The mechanism is similar to the one used in content_shell,
> > > which should allow us to easily port tests in that environment.
> > > 
> > > Review-Url: https://codereview.chromium.org/2842843005
> > > Cr-Original-Commit-Position: refs/heads/master@{#44908}
> > > Bug: 
> > > Change-Id: I9deee0d256a600c60b42902fc8ef8478e5546344
> > > Reviewed-on: https://chromium-review.googlesource.com/494968
> > > Commit-Queue: Mircea Trofin <mtrofin@google.com>
> > > Reviewed-by: Jochen Eisinger <jochen@chromium.org>
> > > Cr-Commit-Position: refs/heads/master@{#45165}
> > 
> > TBR=bradnelson@chromium.org,mtrofin@chromium.org,mtrofin@google.com,jochen@chromium.org
> > NOPRESUBMIT=true
> > NOTREECHECKS=true
> > NOTRY=true
> > 
> > Change-Id: Iafec2615d705d1990c57229cab3a988c00b5e12f
> > Reviewed-on: https://chromium-review.googlesource.com/498630
> > Reviewed-by: Michael Achenbach <machenbach@chromium.org>
> > Commit-Queue: Michael Achenbach <machenbach@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#45166}
> 
> TBR=bradnelson@chromium.org,machenbach@chromium.org,mtrofin@chromium.org,mtrofin@google.com,jochen@chromium.org,v8-reviews@googlegroups.com
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> 
> Change-Id: Ic3c782e918326e291a6cb9bb349c609e9a340b09
> Reviewed-on: https://chromium-review.googlesource.com/498430
> Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
> Commit-Queue: Mircea Trofin <mtrofin@google.com>
> Cr-Commit-Position: refs/heads/master@{#45172}

TBR=bradnelson@chromium.org,machenbach@chromium.org,mtrofin@chromium.org,mtrofin@google.com,jochen@chromium.org,v8-reviews@googlegroups.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Change-Id: I21ffba7141db0bfb4a3275b6e1bf4fb399800ed2
Reviewed-on: https://chromium-review.googlesource.com/500128Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45177}