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

src/property.h

Index: src/property.h
diff --git a/src/property.h b/src/property.h
index aa851f1c88a468a68f3540f2ceee56a085dc2ff8..8fb4b29a198a6fd34dc72af953a2d8ed208a0791 100644
--- a/src/property.h
+++ b/src/property.h
@@ -173,8 +173,10 @@ bool IsPropertyDescriptor(T* desc) {
     }
     case MAP_TRANSITION:
     case CONSTANT_TRANSITION:
-    case NULL_DESCRIPTOR:
       return false;
+    case NONEXISTENT:
+      UNREACHABLE();
+      break;
   }
   UNREACHABLE();  // keep the compiler happy
   return false;