Let svg event attributes be reflected the same as html event attributes.

Corresponds to "SVGElement implements GlobalEventHandlers", https://svgwg.org/svg2-draft/types.html#InterfaceSVGElement.

BUG=338288

Review URL: https://codereview.chromium.org/144733003

git-svn-id: svn://svn.chromium.org/blink/trunk@166087 bbb929c8-8fbe-4397-9dbb-9b2b20218538