[heap] Add a TEAR_DOWN state to heap states which is set when the heap is tearing down.

Bug: chromium:818585
Change-Id: I31032ba230ee2eb9f0bbcab0af720e23c3d6ae11
Reviewed-on: https://chromium-review.googlesource.com/962428Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51955}

src/heap/heap.cc

@@ -5917,6 +5917,7 @@ void Heap::RegisterExternallyReferencedObject(Object** object) {
 }
 
 void Heap::TearDown() {
+  SetGCState(TEAR_DOWN);
   DCHECK(!use_tasks_);
 #ifdef VERIFY_HEAP
   if (FLAG_verify_heap) {

src/heap/heap.h

@@ -599,7 +599,13 @@ class Heap {
 
   enum FindMementoMode { kForRuntime, kForGC };
 
-  enum HeapState { NOT_IN_GC, SCAVENGE, MARK_COMPACT, MINOR_MARK_COMPACT };
+  enum HeapState {
+    NOT_IN_GC,
+    SCAVENGE,
+    MARK_COMPACT,
+    MINOR_MARK_COMPACT,
+    TEAR_DOWN
+  };
 
   using PretenuringFeedbackMap = std::unordered_map<AllocationSite*, size_t>;