Support passing arbitrary messages into amp-iframe

[ericlindley-g]

Primary goal from format perspective is to support more interactivity between parent page and iframed content. Would be useful for highly interactive journalism (scroll-driven storytelling, data visualizations toggled by page-level filters), but is expected to have a wide range of applications beyond that.

Might also consider taking arbitrary messages passed * out of * amp-iframe, but would need to make sure these aren't executed as code in any way that violates the assumptions of AMP

/cc @aghassemi @choumx for thoughts

[aghassemi]

very similar to #9074 same approach can work for both use-cases.

[ericlindley-g]

Ah yes — I had seen that but forgot we were already tracking. Will close this as dupe

[aghassemi]

updated #9074 to include description of this as well as a goal.