[SVG2] The exposed event target should be the <use> element.

Spec:
https://svgwg.org/svg2-draft/struct.html#UseElement

BUG=382872, 313438

Review URL: https://codereview.chromium.org/323343004

git-svn-id: svn://svn.chromium.org/blink/trunk@176090 bbb929c8-8fbe-4397-9dbb-9b2b20218538