Add a test for the mutability of the ShadowRealm global prototype chain

webidl/ecmascript-binding/global-mutable-prototype.any.js

@@ -0,0 +1,27 @@
+// META: global=shadowrealm
+// META: title=Mutability of the global prototype chain
+
+const objects = [];
+setup(() => {
+  for (let object = self; object; object = Object.getPrototypeOf(object)) {
+    objects.push(object);
+  }
+});
+
+test(() => {
+  for (const object of objects) {
+    const proto = Object.getPrototypeOf(object);
+    const plainObject = {};
+    Object.setPrototypeOf(object, plainObject);
+    assert_equals(Object.getPrototypeOf(object), plainObject);
+    Object.setPrototypeOf(object, proto);
+  }
+}, "Setting to a different prototype");
+
+test(() => {
+  for (const object of objects) {
+    const expected = Object.getPrototypeOf(object);
+    Object.setPrototypeOf(object, expected);
+    assert_equals(Object.getPrototypeOf(object), expected);
+  }
+}, "Setting to the same prototype");