Do not implement EventTarget on SVGElementInstance

Do not implement EventTarget on SVGElementInstance, since SVGElementInstance is scheduled for removal and event handling should be handled in the shadow tree. So instead set the shadow tree elements as event target.

BUG=313438

Review URL: https://codereview.chromium.org/257033002

git-svn-id: svn://svn.chromium.org/blink/trunk@175307 bbb929c8-8fbe-4397-9dbb-9b2b20218538