Support `Document.head`

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`?

Start with the discussion in pull request #407, especially the linked comment, and review the issue's questions about Document.head, modifying it in a Worker, style encapsulation, and Document.documentElement. This issue is ready only after the project decides the intended behavior and defines what support or interim behavior counts as done.

Written by the indexing model from the issue text.