Allow expanding variables in amp-iframe src URL

[0x7f]

Description

There are a couple of places where variables like CANONICAL_URL are expanded automatically (form inputs, URL in amp-consent, variables in amp-analytics, etc.). But it is not possible to pass variables to the amp-iframe's src URL. Is there a reason that this is not the case? Or was there just no need for it so far?

I planned to implement it, but faced a few questions:

• Implement it for amp-iframe v0.1 or v1.0 or both?
• The AmpIframe.transformSrc_ function (assuming v0.1 is fine) is a perfect place to put the call to Services.urlReplacementsForDoc. But it is sync. Is it okay to use expandUrlSync and miss the async variables for now? Or shall the whole init code of amp-iframe be changed into async so expandUrlAsync could be used?
• Some variables that should be blacklisted/whitelisted?

Alternatives Considered

I don't really see an alternative for e.g. passing canonical URL to an iframe other than having support for variables.

Additional Context

No response

[erwinmombay]

i believe there was an actual reason for this, but can't remember. Let me dig up some design docs and get back to you.