Adds a FlatDomTreeExtractor to headless

This is very similar to DomTreeExtractor (which we should remove
later) except it uses DOM.getFlattenedDocument.  This is useful for
very large documents which might otherwise blow up the JSON parse
which currently has a nesting depth limit of 200

BUG=546953

Review-Url: https://codereview.chromium.org/2673773002
Cr-Commit-Position: refs/heads/master@{#447907}
5 files changed