Assert that style element has an owner in parse_own_css (#39458)

The existing `assert!(node.is_connected())` is wrong. What it *wants* to
assert is that the style element has an owner, which is either a
Document or a ShadowRoot that the element is a descendant of. However,
if the element is descendant of a ShadowRoot which is itself not
connected to a document then the assertion would fail.

Instead, we use `node.is_in_a_document_tree() ||
node.is_in_a_shadow_tree()`, which more accurately reflects the intent.

Testing: This change adds the test case from
https://github.com/servo/servo/issues/37781 as a crashtest
Fixes https://github.com/servo/servo/issues/39457
Fixes https://github.com/servo/servo/issues/37781

---------

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

tests/wpt/meta/MANIFEST.json

@@ -10380,6 +10380,13 @@
        {}
       ]
      ],
+     "modify-style-element-in-disconnected-shadow-crash.html": [
+      "3808642a97451eef40573214fbe1d1c6273c557f",
+      [
+       null,
+       {}
+      ]
+     ],
      "move-to-new-tree-1343016.html": [
       "853884b99301e22383e0fd0936cf154beab5e93e",
       [