fixed by https://codereview.chromium.org/2119063002/ ?

For headless shell I don't think we're planning on supporting the webserver that implements /json/new, although I'd imagine chrome.exe --headless would have it by default.

Instead we have the new Browser.createTarget and Browser.createBrowserContext devtools protocol commands implemented by that patch. NB I think the ability to have isolated request contexts will be useful for webdriver style testing since it's very much easier now to open a tab with known state.

Gotcha. So you support this at the protocol level. 
We (Lighthouse) can work with this.

Pavel mentions it is probably worth it to support the /json/new endpoint, mostly for backwards compatibility. Telemetry and other protocol users will expect it. He indicates it may be quite straightforward: e.g. kicking off createTarget from the delegate.

I *think* we have everything necessary for tab creation in the protocol now.

 Issue 674034  has been merged into this issue.