shadow-dom: Use static getter to define disabledFeatures (#19479)

This allows shadow-dom/Element-interface-attachShadow-custom-element.html to run in Safari (it currently fails).
diff --git a/shadow-dom/Element-interface-attachShadow-custom-element.html b/shadow-dom/Element-interface-attachShadow-custom-element.html
index 5d862c4..123d9c1 100644
--- a/shadow-dom/Element-interface-attachShadow-custom-element.html
+++ b/shadow-dom/Element-interface-attachShadow-custom-element.html
@@ -20,7 +20,7 @@
 
 test(() => {
   class ShadowDisabledElement extends HTMLElement {
-    static disabledFeatures = ['shadow'];
+    static get disabledFeatures() { return ['shadow']; }
   }
 
   // No definition. attachShadow() should succeed.
@@ -47,7 +47,7 @@
 
 test(() => {
   class ShadowDisabledHeadingElement extends HTMLHeadingElement {
-    static disabledFeatures = ['shadow'];
+    static get disabledFeatures() { return ['shadow']; }
   }
 
   // No definition. attachShadow() should succeed.
@@ -77,7 +77,7 @@
 
 test(() => {
   class CapitalShadowDisabledElement extends HTMLElement {
-    static disabledFeatures = ['SHADOW'];
+    static get disabledFeatures() { return ['shadow']; }
   }
 
   customElements.define('capital-shadow-disabled-element', CapitalShadowDisabledElement);