Use atomic operation to read the length of a fixed array.

src/runtime/runtime-regexp.cc

Index: src/runtime/runtime-regexp.cc
diff --git a/src/runtime/runtime-regexp.cc b/src/runtime/runtime-regexp.cc
index 5846881f90ad5b68505a000753d8fd2f7324ce5f..33c9af9bd401764e80cb3a1ac20a417a18b006a8 100644
--- a/src/runtime/runtime-regexp.cc
+++ b/src/runtime/runtime-regexp.cc
@@ -638,7 +638,7 @@ MUST_USE_RESULT static Object* StringReplaceGlobalRegExpWithEmptyString(
   // thread can not get confused with the filler creation. No synchronization
   // needed.
   heap->CreateFillerObjectAt(end_of_string, delta);
-  heap->AdjustLiveBytes(answer->address(), -delta, Heap::FROM_MUTATOR);
+  heap->AdjustLiveBytes(answer->address(), -delta, Heap::SWEEPING_STARTED);
   return *answer;
 }