Amp-next-page not getting the refeererDocument from the Host Page

## What's the issue?

When giving a pageview from an amp-analytics tag inside a amp-next-page, the documentReferrer should be the hostPage, but it's empty.

## How do we reproduce the issue?

Here you have an example: https://output.jsbin.com/gujosag. Try it in mobile ans let the amp-next-page build. You'll see two calls in the network to ?ping. In ref you have the ${documentReferrer] variable, it's empty on the pageviw of the amp-next-page where it should be the host page.

## What browsers are affected?

All.

## Which AMP version is affected?

All.

Start with the linked JSBin reproduction and inspect the amp-next-page and amp-analytics implementations to trace documentReferrer during the second pageview. Reproduce the two ?ping network calls on mobile and verify that the amp-next-page request reports the host page as its referrer.

Written by the indexing model from the issue text.