[w3ctag/design-reviews] Incubation: FedCM: Support showing third-party iframe origins in the UI (Issue #1136)

cbiesinger created an issue (w3ctag/design-reviews#1136)

### Explainer

https://github.com/w3c-fedid/FedCM/issues/449#issuecomment-1515631336

### The explainer

- [x] Includes the information requested by the [Explainer Explainer](https://w3ctag.github.io/explainer-explainer/#introduction).
- [x] Follows the [Web Platform Design Principles](https://www.w3.org/TR/design-principles/).
- [ ] Includes or links to answers to the [Security/Privacy Questionnaire](https://www.w3.org/TR/security-privacy-questionnaire/).
- [ ] Describes user research you did to validate the problem and/or design.

### Where and by whom is the work is being done?

- GitHub repo: https://github.com/w3c-fedid/FedCM
- Primary contacts:
  - Christian Biesinger (@cbiesinger), Google, Engineer
  - Yi Gu (@yi-gy), Google, Engineer
  - Nicolás Peña Moreno (@npm1), Google, Spec editor/engineer
- Organization/project driving the design: Google
- This work is being funded by: Google
- Incubation and standards groups that have discussed the design:
  - [FedID WG](https://github.com/w3c-fedid/meetings/blob/main/2025/2025-05-20-FedCM-notes.md)
  - [FedID WG](https://github.com/w3c-fedid/meetings/blob/main/2025/2025-06-24-FedCM-notes.md)
  - [FedID WG](https://github.com/w3c-fedid/meetings/blob/main/2025/2025-07-29-FedCM-notes.md)
- Standards group(s) that you expect to discuss and/or adopt this work when it's
  ready: FedID WG


### Feedback so far

- Multi-stakeholder feedback:
  - Chromium comments: n/a
  - Mozilla comments: bvandersloot's comments in https://github.com/w3c-fedid/FedCM/issues/725
  - WebKit comments: no comments so far
- Major unresolved issues with or opposition to this design:

Major opposition is Mozilla's opposition in the issue mentioned above. I would appreciate the TAG's view on this issue, especially on https://github.com/w3c-fedid/FedCM/issues/725#issuecomment-3053927241

### You should also know that...

_No response_

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