DOM: Support atomic moves across shadow boundary

Per discussion with DOM editors, we've resolved to enable this
functionality.

R=nrosenthal@chromium.org

Bug: 40150299
Change-Id: I9bdb577c7538ca9cfe7af95b6c66159ade8e0c72
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5891771
Reviewed-by: Noam Rosenthal <nrosenthal@chromium.org>
Commit-Queue: Dominic Farolino <dom@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1360104}

dom/nodes/moveBefore/tentative/css-transition-cross-shadow.html

@@ -58,7 +58,7 @@
       const shadowContainer = document.querySelector("#shadow-container");
       shadowContainer.shadowRoot.querySelector("#new-parent").moveBefore(item, null);
       await new Promise(resolve => requestAnimationFrame(() => resolve()));
-      assert_greater_than(item.getBoundingClientRect().x, 399);
-    }, "Moving an element with a transition across shadow boundaries should reset the transition");
+      assert_less_than(item.getBoundingClientRect().x, 400);
+    }, "Moving an element with a transition across shadow boundaries preserves the transition");
   </script>
 </body>