[SVG] Handle beginEvent for svg animations

Handled beginEvent for svg animations according to http://www.w3.org/TR/SMIL3/smil-timing.html#q135

R=pdr@chromium.org, schenney@chromium.org, dschulze@chromium.org, fmalita@chromium.org
BUG=269642

Review URL: https://chromiumcodereview.appspot.com/22706003

git-svn-id: svn://svn.chromium.org/blink/trunk@157026 bbb929c8-8fbe-4397-9dbb-9b2b20218538