Make transaction field non-null in IDBObjectStore (#38834)

In the `IDBObjectStore::new` constructor, the `transaction` field is
initialized to null, but when using this constructor, we always execute
`set_transaction` immediately afterward. Therefore, we refactored to
require the `transaction` field to be specified during construction and
thereby also removed some no longer necessary assertions.

We also updated the `transaction` field in WebIDL to remove the nullable
capability.

Testing: WPT
Fixes: #38814

---------

Signed-off-by: criskell <96352451+criskell@users.noreply.github.com>

components/script/dom/idbtransaction.rs

@@ -236,8 +236,8 @@ impl IDBTransactionMethods<crate::DomTypeHolder> for IDBTransaction {
                 name,
                 None,
                 CanGc::note(),
+                self,
             );
-            store.set_transaction(self);
             Dom::from_ref(&*store)
         });