Allow metadata and actions to be routed to different frames?

[xxyzzzq]

As discussed in https://crbug.com/709829, we could probably make the spec more powerful by allowing metadata and actions to be routed to different frames, even different controls can be routed to different frames.

This will be useful in the following scenarios:

1. When the embedder wants to override the metadata in the iframe
2. When the iframe does not support some actions, but the embedder can do that (such as previous/next track)

[youennf]

We probably want to integrate with feature policy here to allow delegating to third party iframes.
I would be tempted to keep this in v1 if there is enough interest from implementors. Otherwise, I would classify it as enhancement for now.
@alastor0325 , @steimelchrome, WDYT.

[youennf]

Marking tentatively as P2, let's consider it once we have a plan for #221