[d8] Make Realm.dispose() trigger a ContextDisposedNotification

Review URL: https://codereview.chromium.org/1129543002

Cr-Commit-Position: refs/heads/master@{#28230}

src/d8.cc

@@ -501,6 +501,8 @@ void Shell::RealmDispose(const v8::FunctionCallbackInfo<v8::Value>& args) {
     return;
   }
   data->realms_[index].Reset();
+  isolate->ContextDisposedNotification();
+  isolate->IdleNotificationDeadline(g_platform->MonotonicallyIncreasingTime());
 }