Fix debug builds.

src/vm-state.h

Index: src/vm-state.h
diff --git a/src/vm-state.h b/src/vm-state.h
index a188c5d2f830a7143fc51447a989735b5f3989fa..241df4c9d48cbb36ce5f84378f7594ab831cbc5c 100644
--- a/src/vm-state.h
+++ b/src/vm-state.h
@@ -42,6 +42,11 @@ class VMState BASE_EMBEDDED {
     external_callback_ = external_callback;
   }
 
+  // Used for debug asserts.
+  static bool is_outermost_external() {
+    return current_state_ == NULL;
+  }
+
   static StateTag current_state() {
     return current_state_ ? current_state_->state() : EXTERNAL;
   }