This reverts commit 3f6686c2.

Reason for revert: https://luci-milo.appspot.com/buildbot/client.v8/V8%20Mac64%20GC%20Stress/196

Original change's description:
> [heap] Initialize the heap tear down at the beginning of Isolate::Deinit
> 
> Code in Isolate::Deinit may trigger a GC, e.g. wasm_engine()->TearDown.
> However, the gin platform in Chrome does not allow to post tasks within
> Isolate::Deinit. By initializing heap tear down at the beginning of
> Isolate::Deinit, we can make that no tasks are posted anymore within
> Isolate::Deinit.
> 
> R=​ulan@chromium.org
> 
> Bug: chromium:826105
> Change-Id: I246c324aa23efe82cc8e7059a1cae5efca33a1b0
> Reviewed-on: https://chromium-review.googlesource.com/983598
> Commit-Queue: Andreas Haas <ahaas@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#52283}

TBR=ulan@chromium.org,ahaas@chromium.org

Change-Id: I98461449b16ae8dcf3b03c51daec92df9f5f6366
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:826105
Reviewed-on: https://chromium-review.googlesource.com/984193Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52284}