[heap] Make SlotSet allocation thread-safe and refactor code.

src/heap/spaces-inl.h

Index: src/heap/spaces-inl.h
diff --git a/src/heap/spaces-inl.h b/src/heap/spaces-inl.h
index 2cbc835d5e4e863e92abbb98050bac9170e4f2f5..937afc694a5b482af6fc8a4b22c991c2d7e72007 100644
--- a/src/heap/spaces-inl.h
+++ b/src/heap/spaces-inl.h
@@ -326,16 +326,16 @@ void Page::MarkNeverAllocateForTesting() {
 
 void Page::MarkEvacuationCandidate() {
   DCHECK(!IsFlagSet(NEVER_EVACUATE));
-  DCHECK_NULL(old_to_old_slots_);
-  DCHECK_NULL(typed_old_to_old_slots_);
+  DCHECK_NULL(slot_set<OLD_TO_OLD>());
+  DCHECK_NULL(typed_slot_set<OLD_TO_OLD>());
   SetFlag(EVACUATION_CANDIDATE);
   reinterpret_cast<PagedSpace*>(owner())->free_list()->EvictFreeListItems(this);
 }
 
 void Page::ClearEvacuationCandidate() {
   if (!IsFlagSet(COMPACTION_WAS_ABORTED)) {
-    DCHECK_NULL(old_to_old_slots_);
-    DCHECK_NULL(typed_old_to_old_slots_);
+    DCHECK_NULL(slot_set<OLD_TO_OLD>());
+    DCHECK_NULL(typed_slot_set<OLD_TO_OLD>());
   }
   ClearFlag(EVACUATION_CANDIDATE);
   InitializeFreeListCategories();