Add onanimationXXX attributes on HTML elements.

Add onanimationXXX attributes on HTML elements to match the prefixed
attributes onwebkitanimationXXX. As it uses the same plumbing as a
regular event listeners, it behaves the same which means that if
onanimationXXX only is defined then only the code attached to this
attribute will be called, if only onwebkitanimationXXX is defined then
only the code attached to this attribute will be called and finally if
both attributes are defined then only the code attached to the
unprefixed attribute will be called.

BUG=234611

Review URL: https://chromiumcodereview.appspot.com/23583032

git-svn-id: svn://svn.chromium.org/blink/trunk@157121 bbb929c8-8fbe-4397-9dbb-9b2b20218538