Use self's pendingTransaction object when setting position.

When iterating through each child to set its updated position,
the transaction object was passed to the children. This could allow
multiple transactions to update the same property on the same child.
If that occurs, then the merge does not guarantee the same order.
This could cause earlier transactions to overwrite later ones. Instead,
have each WC update its own transaction object.

Change-Id: Ie1bb758e4efd74fa012c7ec945d9c1c586162df4
Fixes: 72198558
Fixes: 72463091
Test: Issues from bugs no longer occur.
Test: go/wm-smoke-auto