[heap] Reland Concurrently free empty typed slot set chunks.

src/heap/spaces.h

Index: src/heap/spaces.h
diff --git a/src/heap/spaces.h b/src/heap/spaces.h
index 41e0727a3fbe9a634dff2bde325fd996b9951de9..80f89f23ce563469cadf7c0b06f786c373ac2c72 100644
--- a/src/heap/spaces.h
+++ b/src/heap/spaces.h
@@ -456,7 +456,7 @@ class MemoryChunk {
   inline SlotSet* old_to_new_slots() { return old_to_new_slots_; }
   inline SlotSet* old_to_old_slots() { return old_to_old_slots_; }
   inline TypedSlotSet* typed_old_to_new_slots() {
-    return typed_old_to_new_slots_;
+    return typed_old_to_new_slots_.Value();
   }
   inline TypedSlotSet* typed_old_to_old_slots() {
     return typed_old_to_old_slots_;
@@ -656,7 +656,7 @@ class MemoryChunk {
   // is ceil(size() / kPageSize).
   SlotSet* old_to_new_slots_;
   SlotSet* old_to_old_slots_;
-  TypedSlotSet* typed_old_to_new_slots_;
+  base::AtomicValue<TypedSlotSet*> typed_old_to_new_slots_;

[Hannes Payer (out of office), 2016/09/22 12:49:23]

Let's play safe here an make that an atomic variable since it is read by both
threads

   TypedSlotSet* typed_old_to_old_slots_;
 
   SkipList* skip_list_;