DevTools: editing an inline event removes it and adding it back doesn't work

BUG=329233
R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/169553003

git-svn-id: svn://svn.chromium.org/blink/trunk@167285 bbb929c8-8fbe-4397-9dbb-9b2b20218538