Pass WPT tests for ResizeObserver

[gterzian]

Follow-up to #31108

Folder is wpt/tests/resize-observer.

• calculate-depth-for-node.html requires attachShadow.
• devicepixel.html requires dealing with device pixel calculations(see https://github.com/servo/servo/pull/31108/files#r1470037204).
• eventloop.html fails in relation to Raf delivery.
• /resize-observer/change-layout-in-error.html fails in relation to Raf delivery.
• fragments.html doesn't support more than one fragment
• notify.html need to notify the observer when child node appended and removed.
• observe.html requires Implement HTMLImageElement decode #31183 making the full suite pass. At least one test(test7?) appears intermittently to timeout, and when it passes it makes other test fails(as opposed to not run). See Fix DOMRect calc in ResizeObserverEntry #32549 for how to make at least three tests pass. Others remain to un-investigated.
• ordering.html requires Implement IntersectionObserver #31021
• scrollbars-2.html calculations are wrong
• scrollbars.html calculations are wrong
• svg-with-css-box-001.html need to observe svg
• svg.html need to observe svg
• iframe-same-origin.html appears intermittent for an unknown reason(I haven't seen it fail locally yet, making it hard to debug so far). See Mark some ResizeObserver tests as flaky #32546

[gterzian]

Regarding some of the layout queries to make additonal tests pass, see the comment:

Device_pixel_content_box should snap to device pixels. Also, bounding_content_box_or_zero is the wrong choice if there are multiple fragments, we don't want the bounding rect. As per the current spec it should just be the 1st fragment, with the idea of extending to an array with all fragments in the future.

[gterzian]

Strategy for making all tests pass:

1. Tests that "fail in relation to rafs" can only be fixed after Fix HTML event-loop #31242
2. All other tests can probably be fixed before.