LibWeb/DOM: Create TreeWalker in document's realm

Author: https://github.com/shannonbooth
Commit: 17c92d7c2b
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3208
Reviewed-by: https://github.com/AtkinsSJ ✅

Tests/LibWeb/Text/expected/wpt-import/webidl/current-realm.txt

@@ -2,8 +2,8 @@ Harness status: OK
 
 Found 33 tests
 
-29 Pass
-4 Fail
+30 Pass
+3 Fail
 Pass	querySelectorAll
 Pass	createElement
 Pass	createElementNS
@@ -16,7 +16,7 @@ Pass	createAttributeNS
 Pass	createEvent
 Pass	createRange
 Pass	createNodeIterator
-Fail	createTreeWalker
+Pass	createTreeWalker
 Pass	Request
 Pass	Response
 Fail	getElementsByTagName