Cross-posting from headless-dev:

"IIUC, it's caused by HeadlessDevToolsManagerDelegate doesn't implement CreateNewTarget, when DevToolsHttpHandler::OnJsonRequest receives the request, it calls DevToolsManagerDelegate::CreateNewTarget, which always returns NULL."

We can probably hook up HeadlessDevToolsManagerDelegate::CreateNewTarget to HeadlessDevToolsManagerDelegate::CreateTarget with reasonable defaults for the additional parameters.