script: Get scroll offsets from layout (#37509)

No longer store scroll offsets for elements in the DOM. Instead
consistently get and set these in layout's `ScrollTree`. This more
consistently requires layout to run when querying scroll offsets, which
ensures that they are up-to-date and properly bounded by scrollable
overflow area.

Testing: This causes several WPT tests to start passing, and one to
start
failing. In the case of
`/shadow-dom/scroll-to-the-fragment-in-shadow-tree.html`, I believe the
issue
is that we don't properly handle scrolling and shadow DOM elements.
Before, the
faulty scrolling was hiding this issue.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/shared/compositing/display_list.rs

@@ -424,6 +424,15 @@ impl ScrollTree {
             _ => None,
         }))
     }
+
+    /// Get the scroll offset for the given [`ExternalScrollId`] or `None` if that node cannot
+    /// be found in the tree.
+    pub fn scroll_offset(&self, id: ExternalScrollId) -> Option<LayoutVector2D> {
+        self.nodes.iter().find_map(|node| match node.info {
+            SpatialTreeNodeInfo::Scroll(ref info) if info.external_id == id => Some(info.offset),
+            _ => None,
+        })
+    }
 }
 
 /// In order to pretty print the [ScrollTree] structure, we are converting