LibWeb: Use unsafe layout/paintable accessors where appropriate

Add unsafe_layout_node(), unsafe_paintable(), and unsafe_paintable_box()
accessors that skip layout-staleness verification. These are for use in
contexts where accessing layout/paintable data is legitimate despite
layout not being up to date: tree construction, style recalculation,
painting, animation interpolation, DOM mutation, and invalidation
propagation.

Also add wrapper APIs on Node to centralize common patterns:
- set_needs_display() wraps if (unsafe_paintable()) ...set_needs_display
- set_needs_paint_only_properties_update() wraps similar
- set_needs_layout_update() wraps if (unsafe_layout_node()) ...

And add Document::layout_is_up_to_date() which checks whether layout
tree update flags are all clear.

Libraries/LibWeb/ResizeObserver/ResizeObserverSize.cpp

@@ -28,8 +28,11 @@ ResizeObserverSize::RawSize ResizeObserverSize::compute_box_size(DOM::Element& t
 
     // FIXME: If target is an SVGGraphicsElement that does not have an associated CSS layout box:
     // Otherwise:
-    if (target.paintable_box()) {
-        auto const& paintable_box = *target.paintable_box();
+    // NB: Layout was up to date when observations were gathered, but a previous
+    //     observer's callback may have invalidated it before we get here.
+    //     This matches the behavior of all major browsers.
+    if (target.unsafe_paintable_box()) {
+        auto const& paintable_box = *target.unsafe_paintable_box();
         switch (observed_box) {
         case Bindings::ResizeObserverBoxOptions::BorderBox:
             size.inline_size = paintable_box.border_box_width().to_double();