In-place shrinking of descriptor arrays with non-live transitions.

src/mark-compact.cc

Index: src/mark-compact.cc
diff --git a/src/mark-compact.cc b/src/mark-compact.cc
index 67f6e8e0a7556c89a1f5162ac441ec3da5b4a1c4..d2dce7475ff61d483310926f595b75ac3fe0755d 100644
--- a/src/mark-compact.cc
+++ b/src/mark-compact.cc
@@ -1890,8 +1890,7 @@ void Marker<T>::MarkDescriptorArray(DescriptorArray* descriptors) {
   }
 
   // If the descriptor contains a transition (value is a Map), we don't mark the
-  // value as live. It might be set to the NULL_DESCRIPTOR in
-  // ClearNonLiveTransitions later.
+  // value as live. It might be removed by ClearNonLiveTransitions later.
   for (int i = 0; i < descriptors->number_of_descriptors(); ++i) {
     Object** key_slot = descriptors->GetKeySlot(i);
     Object* key = *key_slot;
@@ -1929,7 +1928,9 @@ void Marker<T>::MarkDescriptorArray(DescriptorArray* descriptors) {
         break;
       case MAP_TRANSITION:
       case CONSTANT_TRANSITION:
-      case NULL_DESCRIPTOR:
+        break;
+      case NONEXISTENT:
+        UNREACHABLE();
         break;
     }
   }