Reland "PaymentRequest: Make displayItems optional"

Reason for revert: Suspect this is causing a failure in the Android CFI
build: https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Android%20CFI/4517

Original change's description:
> PaymentRequest: Make displayItems optional
>
> Before this patch, when calling updateWith() w/o displayItems member in
> PaymentRequestUpdateEvent, it will reset display item list in order
> summary view. This is a implementation bug and doesn't match behavior
> with the spec[1]. So, this patch makes the displayItems member optional
> when updateWith() called.
>
> [1] https://w3c.github.io/payment-request/#update-a-paymentrequest-s-details-algorithm
>
> Bug: 902291
> Change-Id: Ibdf1bf19f38500bd6c559dc507e68e1ad94fbbec
> Reviewed-on: https://chromium-review.googlesource.com/c/1460594
> Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
> Commit-Queue: Jinho Bang <jinho.bang@samsung.com>
> Cr-Commit-Position: refs/heads/master@{#633266}

Bug: 902291
Change-Id: I044ce764efee573bf3b6fc0f0b20f6a26917abea
Reviewed-on: https://chromium-review.googlesource.com/c/1478667
Commit-Queue: Jinho Bang <jinho.bang@samsung.com>
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634089}
9 files changed