[heap] Make setting use_tasks_ explicit

At the moment the flag is set too late, it is possible that the GC still
tries to post tasks in Isolate::Deinit when the isolate is already
disconnected from the platform, see the referenced bug.

R=ulan@chromium.org

Bug: chromium:810739
Change-Id: Ibcd226cb44cc903f2a46e7cccf682b3938c9d408
Reviewed-on: https://chromium-review.googlesource.com/915942Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51274}

src/heap/heap.cc

@@ -5845,7 +5845,7 @@ void Heap::RegisterExternallyReferencedObject(Object** object) {
 }
 
 void Heap::TearDown() {
-  use_tasks_ = false;
+  DCHECK(!use_tasks_);
 #ifdef VERIFY_HEAP
   if (FLAG_verify_heap) {
     Verify();

src/heap/heap.h

@@ -970,6 +970,8 @@ class Heap {
   // Returns whether SetUp has been called.
   bool HasBeenSetUp();
 
+  void stop_using_tasks() { use_tasks_ = false; }
+
   bool use_tasks() const { return use_tasks_; }
 
   // ===========================================================================

src/isolate.cc

@@ -2608,6 +2608,8 @@ void Isolate::ClearSerializerData() {
 void Isolate::Deinit() {
   TRACE_ISOLATE(deinit);
 
+  // Make sure that the GC does not post any new tasks.
+  heap_.stop_using_tasks();
   debug()->Unload();
 
   if (concurrent_recompilation_enabled()) {