[w3c/payment-request] Rethink "payment request is showing" boolean (#809)

I think we should reconsider how "payment request is showing" boolean works and perhaps leave it to the user agent (or the payment handler, rather). 

For example, in Firefox, for "basic-card", there is no real reason why one shouldn't be able to have multiple payment sheets up for different tabs/windows at the same time. 

However, for payment handlers like Apple Pay, where authentication is required either via an Apple Watch or  an iPhone, it makes sense to only allow one payment sheet at a time (because there is a strong tie between the authentication device and the payment sheet being shown).  

Thus, I think what the spec currently states is incorrect: right now, if Sita A is showing a payment sheet, and Site B tries to show a payment sheet, Site B's payment request gets rejected.  

What I think we want is: if required by the Payment Handler, if Sita A is showing a payment sheet, and Site B tries to show a payment sheet, then Site A's payment gets aborted. 

cc @aestes, @rsolomakhin, @zkoch, @mnoorenberghe... 

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