fix: root element not establishing stacking context (#35390) (#36174)

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #35390 
- [X] There are tests for these changes

[Successful WPT
run](https://github.com/reesmichael1/servo/actions/runs/14097679625)
(which includes the new test files)

(I didn't make the formatting changes intentionally--those came from
`mach format` following `mach test-tidy`.)

---------

Signed-off-by: Michael Rees <mrees@noeontheend.com>

components/layout_2020/fragment_tree/base_fragment.rs

@@ -102,6 +102,8 @@ bitflags! {
         /// and the fragment can be a flex item. This flag is used to cache items during flex
         /// layout.
         const SIZE_DEPENDS_ON_BLOCK_CONSTRAINTS_AND_CAN_BE_CHILD_OF_FLEX_ITEM = 1 << 8;
+        /// Whether or not the node that created this fragment is the root element.
+        const IS_ROOT_ELEMENT = 1 << 9;
     }
 }