Handle getting/setting onload for any element.
Trigger a load event after evaluating a <script>.
Update WPT: script-onload-string is passing now.
update past rustup and use DoesNotBubble instead of Bubbles
auto merge of #4326 : eddyb/servo/script-onload, r=jdm

Without this, facebook.com pages (with their custom module loading system and React.js) silently fail to make progress loading content, forever waiting on load events on the initial set of `<script>` elements.