GitLab

The method to check whether the payment instrument for the specified key is
stored.

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/2844673002
Cr-Commit-Position: refs/heads/master@{#467701}