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/layout/layout_impl.rs

@@ -472,6 +472,14 @@ impl Layout for LayoutThread {
             .scroll_tree
             .set_all_scroll_offsets(scroll_states);
     }
+
+    fn scroll_offset(&self, id: ExternalScrollId) -> Option<LayoutVector2D> {
+        self.stacking_context_tree
+            .borrow_mut()
+            .as_mut()
+            .and_then(|tree| tree.compositor_info.scroll_tree.scroll_offset(id))
+            .map(|scroll_offset| -scroll_offset)
+    }
 }
 
 impl LayoutThread {
@@ -996,7 +1004,7 @@ impl LayoutThread {
             .scroll_tree
             .set_scroll_offset_for_node_with_external_scroll_id(
                 external_scroll_id,
-                offset,
+                -offset,
                 ScrollType::Script,
             )
         {