[SVG2] Add onbegin, onend and onrepeat EventHandlers on SVGAnimationElement

These map to the eventnames 'beginEvent', 'endEvent' and 'repeatEvent'
respectively.

Spec:
https://svgwg.org/svg2-draft/animate.html#InterfaceSVGAnimationElement

BUG=338288
NOTRY=true

Review URL: https://codereview.chromium.org/201673003

git-svn-id: svn://svn.chromium.org/blink/trunk@169871 bbb929c8-8fbe-4397-9dbb-9b2b20218538