[Payment Request] Update the logic when obtaining results from editors
There were bugs in the handling of the result coming back from an editor. We now have improved logic around:
* Receiving |null| from the editor (cancelling out of an Add flow)
* Receiving an incomplete result (cancelling out of an Edit for an incomplete item)
* Receiving a complete item.
Also updated the tests and added a few new ones.
BUG=
673371
TEST=PaymentRequest* (java)
Committed:
https://crrev.com/8824e3d6fcef98df0702cf0c0dd6c3845d067ff6
Cr-Commit-Position: refs/heads/master@{#439038}