Payment Handler enrollment should accept URLs only

[maxlgu]

Now that PaymentRequest API no longer accepts accept non-URL method names, the payment handler enrollment should accept URLs only as well.

[maxlgu]

@rsolomakhin

[rsolomakhin]

Ref: https://groups.google.com/a/chromium.org/g/blink-dev/c/ZS0JxjApEhI/m/GP1_t04gBAAJ