Support SVG in event.path.

The SVG instance tree was recently removed which also removed most of the gnarly event handling in SVG. Now we can support SVG also in event.path.

BUG=402721

Review URL: https://codereview.chromium.org/477883003

git-svn-id: svn://svn.chromium.org/blink/trunk@180364 bbb929c8-8fbe-4397-9dbb-9b2b20218538