Revert "[heap] Initialize the heap tear down at the beginning of Isolate::Deinit"

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}

src/heap/heap.cc

@@ -5991,10 +5991,8 @@ void Heap::RegisterExternallyReferencedObject(Object** object) {
   }
 }
 
-void Heap::StartTearDown() { SetGCState(TEAR_DOWN); }
-
 void Heap::TearDown() {
-  DCHECK_EQ(gc_state_, TEAR_DOWN);
+  SetGCState(TEAR_DOWN);
 #ifdef VERIFY_HEAP
   if (FLAG_verify_heap) {
     Verify();

src/heap/heap.h

@@ -978,9 +978,6 @@ class Heap {
   // Create ObjectStats if live_object_stats_ or dead_object_stats_ are nullptr.
   void CreateObjectStats();
 
-  // Sets the TearDown state, so no new GC tasks get posted.
-  void StartTearDown();
-
   // Destroys all memory allocated by the heap.
   void TearDown();
 

src/isolate.cc

@@ -2625,7 +2625,6 @@ void Isolate::ClearSerializerData() {
 void Isolate::Deinit() {
   TRACE_ISOLATE(deinit);
 
-  heap_.StartTearDown();
   debug()->Unload();
 
   if (concurrent_recompilation_enabled()) {