Refactor HTMLTreeBuilder/HTMLConstructionSite ctor

No behavioral change.

- Avoid duplicate member init in HTMLConstructionSite c-tor.
- Use Document& instead of Document* as the document can't be nullref
- Clarify that HTMLTreeBuilder can be invoked for generic Document (with a fixme comment)

BUG=None

Review-Url: https://codereview.chromium.org/2126043003
Cr-Commit-Position: refs/heads/master@{#404615}
5 files changed