Add a resize observer step to the "update the rendering" steps

[smfr]

Resize Observer is described in a CSS working group draft https://drafts.csswg.org/resize-observer-1/, but is supported by the major browsers now.

https://drafts.csswg.org/resize-observer-1/#html-event-loop has some not-very-clear text about when observerations fire, but written as a confusing delta on the event loop steps https://html.spec.whatwg.org/multipage/webappapis.html#event-loop-processing-model.

https://html.spec.whatwg.org/multipage/webappapis.html#event-loop-processing-model should just add a step for resize observer.

[smfr]

@atotic