[Web Payments] Add a mechanism to build item lists in the PR dialog.

chrome/browser/ui/views/payments/payment_request_views_util.h

Index: chrome/browser/ui/views/payments/payment_request_views_util.h
diff --git a/chrome/browser/ui/views/payments/payment_request_views_util.h b/chrome/browser/ui/views/payments/payment_request_views_util.h
index 424771b086f0f03f6663e22bc350cb1c6163e405..10e23be9343c393fdc3b3e85f8fc43fd77cf72f8 100644
--- a/chrome/browser/ui/views/payments/payment_request_views_util.h
+++ b/chrome/browser/ui/views/payments/payment_request_views_util.h
@@ -25,9 +25,11 @@ namespace payments {
 
 constexpr int kPaymentRequestRowHorizontalInsets = 16;
 constexpr int kPaymentRequestRowVerticalInsets = 8;
+
 // Extra inset relative to the header when a right edge should line up with the
 // close button's X rather than its invisible right edge.
 constexpr int kPaymentRequestRowExtraRightInset = 8;
+constexpr int kPaymentRequestButtonSpacing = 10;
 
 enum class PaymentRequestCommonTags {
   BACK_BUTTON_TAG = 0,