Explainer:
https://github.com/camillelamy/explainers/blob/main/anonymous_iframes.md

Chrome status:
https://chromestatus.com/feature/5729461725036544

Summary:
- Define the anonymous flag for iframe and Window.
- Inheritance is defined similarly to sandbox. However it do not propage
  toward popups.
- Popup opened from anonymous Window use 'noopener'.
- Navigation in anonymous iframe are allowed, even if the embedder has
  COEP:require-corp|credentialless and the response do not.
- Define the `page anonymous nonce`, it is used for anonymous Window as
  an additional keys in:
  - network-partition-keys,
  - storage-partition-keys,
  - cookie-partition-keys
  This ensures the document is loaded within a new and ephemeral
  context. This prevents a cross-origin-isolated parent from stealing
  important data from its child, via a Spectre Attack.
- Password autofill must be disabled inside anonymous Window.

XXX: implement the corresponding parts on top of:
- Fetch => network-partition-keys
- StoragePartitioning => storage-partition-keys
- CookieHavingIndependantState => cookie-partition-key
- Worker.

This reverts commit 5467e4f.