Eliminate unnecessary uses of Node::ToNode

This method is supposed to be used to go from an EventTarget to its node
(if any). Remove uses where it's called on something that obviously
derives from Node already.
Make it private on Node to prevent any future such uses.

Change-Id: I0ed70ac122e877ae90ee7c6f6c53caaa63edd574
Reviewed-on: https://chromium-review.googlesource.com/c/1361070
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#613910}
4 files changed