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
Change-Id: Ie3bac4ec297c0b85c40b45495e9c823dd47cb49e

Author: mfreed7

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

@@ -46,33 +46,43 @@
   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');
-</script>
 
-<div id=open2>
-  <template shadowrootmode=open shadowrootdelegatesfocus shadowrootclonable>
-    Open, delegates focus (not the default), clonable (not the default)
-    named slot assignment (the default)
-  </template>
-</div>
+function getDeclarativeHost(t) {
+  const div = document.createElement('div');
+  div.setHTMLUnsafe(`
+    <template shadowrootmode=open shadowrootdelegatesfocus shadowrootclonable serializable>
+      Open, delegates focus (not the default), clonable (not the default)
+      serializable (not the default), named slot assignment (the default)
+    </template>
+  `);
+  t.add_cleanup(() => div.remove());
+  document.body.appendChild(div);
+  return div;
+}
 
-<script>
 test((t) => {
+  // Changing the type should throw.
+  let host = getDeclarativeHost(t);
   assert_throws_dom("NotSupportedError",() => {
-    open2.attachShadow({mode: "closed", delegatesFocus: true, slotAssignment: "named", clonable: true});
+    host.attachShadow({mode: "closed"});
   },'Mismatched shadow root type should throw');
   assert_throws_dom("NotSupportedError",() => {
-    open2.attachShadow({mode: "open", delegatesFocus: false, slotAssignment: "named", clonable: true});
-  },'Mismatched shadow root delegatesFocus should throw');
-  assert_throws_dom("NotSupportedError",() => {
-    open2.attachShadow({mode: "open", delegatesFocus: true, slotAssignment: "manual", clonable: true});
-  },'Mismatched shadow root slotAssignment should throw');
-  assert_throws_dom("NotSupportedError",() => {
-    open2.attachShadow({mode: "open", delegatesFocus: true, slotAssignment: "named", clonable: false});
-  },'Mismatched shadow root clonable should throw');
+    host.attachShadow({mode: "closed", delegatesFocus: true, slotAssignment: "named", clonable: true, serializable: true});
+  },'Mismatched shadow root type should throw (explicit args)');
 
-  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');
+  // Changing other things should not throw, and should not change the shadow root's settings
+  host = getDeclarativeHost(t);
+  assert_equals(host.delegatesFocus,true);
+  assert_equals(host.slotAssignment,"named");
+  assert_true(host.clonable);
+  assert_true(host.serializable);
+  const initialShadow = host.shadowRoot;
+  let newShadow = host.attachShadow({mode: "open", delegatesFocus: false, slotAssignment: "manual", clonable: false, serializable: false});
+  assert_equals(newShadow,initialShadow,'Same shadow should be returned');
   assert_equals(shadow.textContent,'','Shadow should be empty');
+  assert_equals(host.delegatesFocus,true);
+  assert_equals(host.slotAssignment,"named");
+  assert_true(host.clonable);
+  assert_true(host.serializable);
 },'Calling attachShadow() on declarative shadow root must match all parameters');
 </script>