Fix panic when called on root node, and update test expectations.

The three tests that are marked as failures are because they use offsetWidth as a dummy to force a layout flush. Now that these exist, they expose an unrelated bug with reflow.

components/script/dom/window.rs

@@ -839,8 +839,8 @@ impl<'a> WindowHelpers for &'a Window {
             Some(parent_node_address) => {
                 let node = from_untrusted_node_address(js_runtime.rt(),
                                                        parent_node_address);
-                let element = ElementCast::to_ref(node.r()).unwrap();
-                Some(Root::from_ref(element))
+                let element = ElementCast::to_ref(node.r());
+                element.map(Root::from_ref)
             }
             None => {
                 None

tests/wpt/metadata-css/css21_dev/html4/first-letter-dynamic-003a.htm.ini

@@ -0,0 +1,3 @@
+[first-letter-dynamic-003a.htm]
+  type: reftest
+  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/first-letter-dynamic-003b.htm.ini

@@ -0,0 +1,3 @@
+[first-letter-dynamic-003b.htm]
+  type: reftest
+  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/run-in-basic-017.htm.ini

@@ -0,0 +1,3 @@
+[run-in-basic-017.htm]
+  type: reftest
+  expected: FAIL

tests/wpt/metadata/html/semantics/forms/the-input-element/input-type-button.html.ini

@@ -1,5 +0,0 @@
-[input-type-button.html]
-  type: testharness
-  [label value]
-    expected: FAIL
-