Introduce --zap-cpp-pointers (off by default)

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index b2d21e4e62e5d38b8428cc5b0d643fef2666f746..752b97e04227c63d68498a30551cb2011f4d28c8 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -614,6 +614,11 @@ class Isolate {
   v8::TryCatch* try_catch_handler() {
     return thread_local_top_.try_catch_handler();
   }
+#if DEBUG
+  static v8::TryCatch* next_try_catch(v8::TryCatch* try_catch) {
+    return try_catch->next_;
+  }
+#endif
   bool* external_caught_exception_address() {
     return &thread_local_top_.external_caught_exception_;
   }