Add a flag to d8 to invoke weak callbacks

src/d8.h

Index: src/d8.h
diff --git a/src/d8.h b/src/d8.h
index 2c7513f553dff17a1935a9ee2b736c54467853e2..1f7f5d1ee6bd77bbccfb48688aafcc306cba3b93 100644
--- a/src/d8.h
+++ b/src/d8.h
@@ -198,6 +198,7 @@ class ShellOptions {
      script_executed(false),
      last_run(true),
      send_idle_notification(false),
+     invoke_weak_callbacks(false),
      stress_opt(false),
      stress_deopt(false),
      interactive_shell(false),
@@ -220,6 +221,7 @@ class ShellOptions {
   bool script_executed;
   bool last_run;
   bool send_idle_notification;
+  bool invoke_weak_callbacks;
   bool stress_opt;
   bool stress_deopt;
   bool interactive_shell;