HTMLDetailsElement should fire a 'toggle' event

As per the specification, a 'toggle' event should be fired whenever the 'open'
attribute is added to or removed from a details element:
http://www.whatwg.org/specs/web-apps/current-work/multipage/interactive-elements.html#attr-details-open

Firefox 28 and IE11 do not support this yet.

R=arv@chromium.org, tkent@chromium.org
BUG=366105
TEST=fast/html/details-open-toggle-event.html

Review URL: https://codereview.chromium.org/249573002

git-svn-id: svn://svn.chromium.org/blink/trunk@172427 bbb929c8-8fbe-4397-9dbb-9b2b20218538