Use proper event target rules for SVG content in shadow trees

This patch changes how events are handled for SVG elements in non-use
shadow trees (the use case will be handled by crbug.com/313438).
Instead of exposing the shadow element as the event target we now
use the proper event retargeting code to return the host element.

BUG=370136

Review URL: https://codereview.chromium.org/261323004

git-svn-id: svn://svn.chromium.org/blink/trunk@173398 bbb929c8-8fbe-4397-9dbb-9b2b20218538