Add a flag to track detached contexts.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index d5417acf17ae696d7f83fc3172ea2c83b6bcaab6..08d5e03e10109c526cd38cc15175a3c936ad4f31 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -854,6 +854,9 @@ bool Heap::CollectGarbage(GarbageCollector collector, const char* gc_reason,
     }
 
     GarbageCollectionEpilogue();
+    if (collector == MARK_COMPACTOR && FLAG_track_detached_contexts) {
+      isolate()->CheckDetachedContextsAfterGC();
+    }
     tracer()->Stop(collector);
   }
 
@@ -3071,6 +3074,8 @@ void Heap::CreateInitialObjects() {
     set_keyed_load_dummy_vector(empty_fixed_array());
   }
 
+  set_detached_contexts(empty_fixed_array());
+
   Handle<SeededNumberDictionary> slow_element_dictionary =
       SeededNumberDictionary::New(isolate(), 0, TENURED);
   slow_element_dictionary->set_requires_slow_elements();