The PaymentManager is defined as an extension of ServiceWorkerRegistration (like PushManager).
  - https://w3c.github.io/payment-handler/#extension-to-the-serviceworkerregistration-interface

ServiceWorker is only allowed in SecureContext but Chrome enables ServiceWorker in Extension side exceptionally.
Therefore, the PaymentManager might expose in Extension side together.
To resolve this issue, we should block accessing PaymentManager in Extension side checking https scheme.