Support `Document.head`

[kristoferbaxter]

https://github.com/ampproject/worker-dom/pull/407#discussion_r271966063

Should we support this concept? What would the implementation look like for modifying document.head in a Worker reflected to the main thread? Are styles encapsulated?

Until we decide, should we make Document.documentElement = null?

[dvoytenko]

Possibly some confusion here. But is the document.head support and documentElement have to be related?

I think we can start with a document.head == null for a foreseeable future. But I think document vs document.documentElement case has to be resolved regardless. One is a node and the other one is an element and there are some important distinction that could be embedded into this in a user script.