Add beginEvent/endEvent listeners via the onbegin/onend attributes.

SVG2 spec: https://svgwg.org/svg2-draft/single-page.html#script-OnBeginEventAttribute
SVG 1.1 spec: http://www.w3.org/TR/SVG11/script.html#OnBeginEventAttribute

R=pdr@chromium.org, schenney@chromium.org, dschulze@chromium.org, fmalita@chromium.org
BUG=116595

Review URL: https://codereview.chromium.org/106053007

git-svn-id: svn://svn.chromium.org/blink/trunk@163419 bbb929c8-8fbe-4397-9dbb-9b2b20218538