Only check for matching mode in attachShadow

Per the new-new consensus, attachShadow will only verify that
the existing declarative shadow root's `mode` matches the newly
requested `mode`:

whatwg/html#10107 (comment)

Bug: 41483062,325598615
Change-Id: Ie3bac4ec297c0b85c40b45495e9c823dd47cb49e

shadow-dom/declarative/declarative-shadow-dom-repeats.html

@@ -46,35 +46,48 @@
 test((t) => {
   assert_throws_dom("NotSupportedError",() => {
     open1.attachShadow({mode: "closed"});
-  },'Mismatched shadow root type should throw');
+  },'Mismatched shadow root mode should throw');
   const initialShadow = open1.shadowRoot;
   const shadow = open1.attachShadow({mode: "open"}); // Shouldn't throw
   assert_equals(shadow,initialShadow,'Same shadow should be returned');
   assert_equals(shadow.textContent,'','Shadow should be empty');
-},'Calling attachShadow() on declarative shadow root must match type');
+},'Calling attachShadow() on declarative shadow root must match mode');
 </script>
 
 <div id=open2>
-  <template shadowrootmode=open shadowrootdelegatesfocus shadowrootclonable>
+  <template shadowrootmode=open shadowrootdelegatesfocus shadowrootclonable serializable>
     Open, delegates focus (not the default), clonable (not the default)
-    named slot assignment (the default)
+    serializable (not the default), named slot assignment (the default)
   </template>
 </div>
 
 <script>
 test((t) => {
+  t.add_cleanup(() => open2.remove());
+  assert_true(!!open2.shadowRoot);
+  // Changing the mode should throw.
   assert_throws_dom("NotSupportedError",() => {
-    open2.attachShadow({mode: "closed", delegatesFocus: true, slotAssignment: "named", clonable: true});
+    open2.attachShadow({mode: "closed"});
   },'Mismatched shadow root mode should throw');
+  assert_throws_dom("NotSupportedError",() => {
+    open2.attachShadow({mode: "closed", delegatesFocus: true, slotAssignment: "named", clonable: true, serializable: true});
+  },'Mismatched shadow root mode should throw (explicit args)');
 
+  // Changing other things should not throw, and should not change the shadow root's settings
   const initialShadow = open2.shadowRoot;
-  const shadow = open2.attachShadow({mode: "open", delegatesFocus: true, slotAssignment: "named", clonable: true}); // Shouldn't throw
-  assert_equals(shadow,initialShadow,'Same shadow should be returned');
-  assert_equals(shadow.textContent,'','Shadow should be empty');
-
+  assert_equals(initialShadow.delegatesFocus,true);
+  assert_equals(initialShadow.slotAssignment,"named");
+  assert_true(initialShadow.clonable);
+  assert_true(initialShadow.serializable);
+  let newShadow = open2.attachShadow({mode: "open", delegatesFocus: false, slotAssignment: "manual", clonable: false, serializable: false});
+  assert_equals(newShadow,initialShadow,'Same shadow should be returned');
+  assert_equals(newShadow.textContent,'','Shadow should be empty');
+  assert_equals(newShadow.delegatesFocus,true);
+  assert_equals(newShadow.slotAssignment,"named");
+  assert_true(newShadow.clonable);
+  assert_true(newShadow.serializable);
   assert_throws_dom("NotSupportedError",() => {
     open2.attachShadow({mode: "open"});
   },'Invoking attachShadow() on a non-declarative shadow root should throw');
-
 },'Calling attachShadow() on declarative shadow root must match all parameters');
 </script>