Specify whether to consider ancestor frames when applying allow-same-site-none-cookies

[aamuley]

Based on the conversation in the Mozilla standards positions repo, additionally specify that the ancestor frames should be considered when deciding whether the allow-same-site-none-cookies value should be applied to a sandboxed context.

If 3PCs are blocked, allowing a cross-site embed to access its domain's SameSite=None cookies could be a circumvention of 3PC Blocking.