Spec: move validation of some input parameters to construction time

[stephenmcgruer]

Currently, the spec does all its data validation in the steps to check if a payment can be made, which are called during show() processing (step 16.6.1). This is at odds with the actual implementation in Chrome, which validates a lot of the data during construction, not show() (e.g., see https://wpt.fyi/results/secure-payment-confirmation/constructor.https.html?label=master&label=experimental&aligned).

I believe that validating the data during construction is actually correct (or at least, useful) as it is trivial checking (e.g., If data["credentialIds"] is empty, return false.). There is actually a PaymentRequest issue tracking adding a concept for this - w3c/payment-request#976, for which I have a PR (w3c/payment-request#977) that could address it, but it needs a little TLC.

I would like us to try to get that PR landed, and then update the SPC spec to do some of the validation during construction.