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}