http2: track async state for sending

Sending pending data may involve running arbitrary JavaScript code.
Therefore, it should involve a callback scope.

PR-URL: nodejs/node#16461
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

src/node_http2.cc

@@ -91,6 +91,12 @@ Http2Session::Http2Session(Environment* env,
   prep_->data = static_cast<void*>(this);
   uv_prepare_start(prep_, [](uv_prepare_t* t) {
     Http2Session* session = static_cast<Http2Session*>(t->data);
+    HandleScope scope(session->env()->isolate());
+    Context::Scope context_scope(session->env()->context());
+
+    // Sending data may call arbitrary JS code, so keep track of
+    // async context.
+    InternalCallbackScope callback_scope(session);
     session->SendPendingData();
   });
 }