Removed incorrect NOTREACHED in typedom

The NOTREACHED triggered when accessing shorthands in attributeStyleMap
for elements without a style attribute.

Bug: 1290995
Change-Id: Ia2888db8959ce1350246337b704cc44a50eaccde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3416193
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#963768}

css/css-typed-om/resources/testhelper.js

@@ -144,6 +144,17 @@ function createDivWithStyle(test, cssText) {
   return element;
 }
 
+// Creates a new div element without inline style.
+// The created element is deleted during test cleanup.
+function createDivWithoutStyle(test) {
+  let element = document.createElement('div');
+  document.body.appendChild(element);
+  test.add_cleanup(() => {
+    element.remove();
+  });
+  return element;
+}
+
 // Creates a new div element with inline style |cssText| and returns
 // its inline style property map.
 function createInlineStyleMap(test, cssText) {

css/css-typed-om/the-stylepropertymap/inline/get-shorthand.html

@@ -27,4 +27,10 @@
 }, 'Getting a shorthand property that is partially set in inline style ' +
    'returns null');
 
+test(t => {
+  const styleMap = createDivWithoutStyle(t).attributeStyleMap;
+  assert_equals(styleMap.get('margin'), null,
+    'Shorthand value must be null for element without style');
+}, 'Getting an attributeStyleMap shorthand property from an element without ' +
+   'a style attribute');
 </script>