Fix crash in RespondWithObserver::respondWith

This is the same fix as r166987 for WaitUntilObserver.

No test is added. The upcoming Chromium CL
https://codereview.chromium.org/178343011/ caught
this crash so it will be tested by it.

BUG=342301

Review URL: https://codereview.chromium.org/192603002

git-svn-id: svn://svn.chromium.org/blink/trunk@168833 bbb929c8-8fbe-4397-9dbb-9b2b20218538