Use EventSender class in HTMLSourceElement

Use EventSender class in HTMLSourceElement to avoid duplicating the
asynchronous event dispatch code.

R=esprehn@chromium.org, tkent@chromium.org

Review URL: https://codereview.chromium.org/258503007

git-svn-id: svn://svn.chromium.org/blink/trunk@172560 bbb929c8-8fbe-4397-9dbb-9b2b20218538