Fix scrolling from script in Layout 2020

Script will only scroll if it detects that an element has a scrolling
box, so this change adds an implementation of the scroll area query to
Layout 2020. This allows some scrolling tests to start passing.

This change also updates all expected results in css-backgrounds and
cssom-view.

components/layout_2020/flow/root.rs

@@ -538,6 +538,39 @@ impl FragmentTree {
         })
         .unwrap_or_else(Rect::zero)
     }
+
+    pub fn get_scroll_area_for_node(&self, requested_node: OpaqueNode) -> Rect<i32> {
+        let mut scroll_area: PhysicalRect<Length> = PhysicalRect::zero();
+        let tag_to_find = Tag::Node(requested_node);
+        self.find(|fragment, _, containing_block| {
+            if fragment.tag() != Some(tag_to_find) {
+                return None::<()>;
+            }
+
+            scroll_area = match fragment {
+                Fragment::Box(fragment) => fragment
+                    .scrollable_overflow(&containing_block)
+                    .translate(containing_block.origin.to_vector()),
+                Fragment::Text(_) |
+                Fragment::AbsoluteOrFixedPositioned(_) |
+                Fragment::Image(_) |
+                Fragment::IFrame(_) |
+                Fragment::Anonymous(_) => return None,
+            };
+            None::<()>
+        });
+
+        Rect::new(
+            Point2D::new(
+                scroll_area.origin.x.px() as i32,
+                scroll_area.origin.y.px() as i32,
+            ),
+            Size2D::new(
+                scroll_area.size.width.px() as i32,
+                scroll_area.size.height.px() as i32,
+            ),
+        )
+    }
 }
 
 /// https://drafts.csswg.org/css-backgrounds/#root-background