Revert of PaymentHandler: Implement PaymentInstruments.keys(). (patchset #3 id:40001 of https://codereview.chromium.org/2850203002/ )

Reason for revert:
This patch is causing some PaymentManager tests to fail.

Original issue's description:
> PaymentHandler: Implement PaymentInstruments.keys().
>
> The method to return keys of the stored payment instruments in insertion order.
>
> Related Spec Link:
>   https://w3c.github.io/webpayments-payment-handler/#paymentinstruments-interface
>
> BUG=661608
> TEST=payment_manager_unittest.cc, payment-instruments.html
>
> Review-Url: https://codereview.chromium.org/2850203002
> Cr-Commit-Position: refs/heads/master@{#468747}
> Committed: https://chromium.googlesource.com/chromium/src/+/09c61ecc5adf140aa7a59aef0ecacc510e0be1d8

TBR=rouslan@chromium.org,tsepez@chromium.org,nhiroki@chromium.org,jinho.bang@samsung.com
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=661608

Review-Url: https://codereview.chromium.org/2858773002
Cr-Commit-Position: refs/heads/master@{#468773}
17 files changed