Fix the "get the parent" loop when dispatching event (fixes #6733)

The DOM specification says:

A document's get the parent algorithm, given an event, returns null
if event's type attribute value is "load" or document does not have
a browsing context, and the document's associated Window object
otherwise.

tests/wpt/metadata-css/css-transitions-1_dev/html/before-load-001.htm.ini

@@ -1,3 +1,5 @@
 [before-load-001.htm]
   type: testharness
-  expected: TIMEOUT
+  [transition height from 10px to 100px / events]
+    expected: FAIL
+