Fix index used to join results of DOM.getDocument and CSS.getLayoutTreeAndStyles

I had thought the backend node id matched the output of DOM.getDocument, but it
doesn't.

As an added bonus I've omitted empty css style properties from
the results.

BUG=546953

Review-Url: https://codereview.chromium.org/2455613002
Cr-Commit-Position: refs/heads/master@{#428178}
7 files changed