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 978c52ec2a7d9953f2bda54c6e1a9a9e428fc736..da298efee31dfcf64489cb99ee859537c4ac91fc 100644
--- a/src/isolate.cc
+++ b/src/isolate.cc
@@ -2626,8 +2626,8 @@ 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::CONCURRENT_TO_SWEEPER>(
+        *detached_contexts, length - new_length);
   }
 }