Add DOMModelSnapshot and takeSnapshot method to DOMNode

The snapshot is not registered with a DOMModel and doesn't reflect
live DOM updates


Bug: 475842776
Change-Id: Ia4806cb275e34204e746d9f465a5c2a5ca9323ca
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/7642489
Commit-Queue: Danil Somsikov <dsv@chromium.org>
Auto-Submit: Kateryna Prokopenko <kprokopenko@chromium.org>
Commit-Queue: Kateryna Prokopenko <kprokopenko@chromium.org>
Reviewed-by: Danil Somsikov <dsv@chromium.org>

Author: Kateryna Prokopenko