Introduce a flag for tracing retaining path in GC.

src/isolate.cc

Index: src/isolate.cc
diff --git a/src/isolate.cc b/src/isolate.cc
index 37020da1559b70828dfceedf5eb75cb32391817b..16f5466aef2dc3ee7ec649cb382e84e8462adbee 100644
--- a/src/isolate.cc
+++ b/src/isolate.cc
@@ -2597,8 +2597,8 @@ void Isolate::CheckDetachedContextsAfterGC() {
       new_length += 2;
     }
   }
-  PrintF("%d detached contexts are collected out of %d\n", length - new_length,
-         length);
+  PrintF("%d detached contexts are collected out of %d\n",
+         (length - new_length) / 2, length / 2);
   for (int i = 0; i < new_length; i += 2) {
     int mark_sweeps = Smi::cast(detached_contexts->get(i))->value();
     WeakCell* cell = WeakCell::cast(detached_contexts->get(i + 1));
@@ -2607,9 +2607,9 @@ void Isolate::CheckDetachedContextsAfterGC() {
              static_cast<void*>(cell->value()), mark_sweeps);
     }
   }
-  if (length == new_length) {
+  if (new_length == 0) {
     heap()->set_detached_contexts(heap()->empty_fixed_array());
-  } else {
+  } else if (new_length < length) {
     heap()->RightTrimFixedArray<Heap::FROM_GC>(*detached_contexts,
                                                length - new_length);
   }