Synchronize store buffer processing and concurrent sweeping.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 3939921bf7304378b13540fc4a1fda3f426216b3..d3c8f00c273ffcb9d4b38cf8192b5b1c17d07837 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -2989,6 +2989,7 @@ SMI_ACCESSORS(FixedArrayBase, length, kLengthOffset)
 SYNCHRONIZED_SMI_ACCESSORS(FixedArrayBase, length, kLengthOffset)
 
 SMI_ACCESSORS(FreeSpace, size, kSizeOffset)
+SYNCHRONIZED_SMI_ACCESSORS(FreeSpace, size, kSizeOffset)
 
 SMI_ACCESSORS(String, length, kLengthOffset)
 SYNCHRONIZED_SMI_ACCESSORS(String, length, kLengthOffset)
@@ -4016,7 +4017,7 @@ int HeapObject::SizeFromMap(Map* map) {
     return reinterpret_cast<ByteArray*>(this)->ByteArraySize();
   }
   if (instance_type == FREE_SPACE_TYPE) {
-    return reinterpret_cast<FreeSpace*>(this)->size();
+    return reinterpret_cast<FreeSpace*>(this)->synchronized_size();
   }
   if (instance_type == STRING_TYPE ||
       instance_type == INTERNALIZED_STRING_TYPE) {