[story-ads] Support analytics triggers when an ad is served into a x-domain iframe.

[calebcordry]

Description

Story ads are built on top of the AMPHTML Ads format. Usually this means they are rendered into a friendly iframe in web stories. However there are cases where these creatives are rendered into x-domain iframes, and ad networks may decide to start using x-domain at any time.

When an ad is served into a x-domain iframe analytics triggers on the parent page will not fire. We could find a way to postmessage these events from the creative to the parent page using the inabox library. The parent page could listen for the messages and fire the events appropriately.

Potential issues

• If an ad network has decided to serve a creative x-domain they may not want us to leak data to the publishers. These are policy questions that are very tricky to work out
• When you open the world to x-domain iframes sometimes you will start getting things like nested iframes where it would be difficult to ensure these events are propagated.

@ampproject/wg-stories

Alternatives Considered

Don't support this.

Additional Context

https://github.com/ampproject/amphtml/issues/37831

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