Don't expose the user agent shadow tree

Don't expose the user agent shadow tree used for the cloned <use> tree. Add one test for the attribute event listener code path (onclick) and one for addEventListener API. To make sure the shadow tree is not exposed, make sure we always wrap non-shadow tree SVG Elements.

BUG=313438

Review URL: https://codereview.chromium.org/312423002

git-svn-id: svn://svn.chromium.org/blink/trunk@176065 bbb929c8-8fbe-4397-9dbb-9b2b20218538