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

src/isolate.cc

Index: src/isolate.cc
diff --git a/src/isolate.cc b/src/isolate.cc
index bdf82926ceea02c8663061278c0c3e7846d2c845..87398e7f8fd4b7edfc39415252f54925109ac225 100644
--- a/src/isolate.cc
+++ b/src/isolate.cc
@@ -2632,8 +2632,8 @@ void Isolate::CheckDetachedContextsAfterGC() {
   if (new_length == 0) {
     heap()->set_detached_contexts(heap()->empty_fixed_array());
   } else if (new_length < length) {
-    heap()->RightTrimFixedArray<Heap::FROM_MUTATOR>(*detached_contexts,
-                                                    length - new_length);
+    heap()->RightTrimFixedArray<Heap::SWEEPING_STARTED>(*detached_contexts,
+                                                        length - new_length);
   }
 }