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

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

Index: chrome/browser/ui/views/payments/payment_request_sheet_controller.h
diff --git a/chrome/browser/ui/views/payments/payment_request_sheet_controller.h b/chrome/browser/ui/views/payments/payment_request_sheet_controller.h
index a3d2497752e9139cbd9d1b7edaffecd3063bcba9..b341fc89e9ba2fa28e950d029caeb7736b1ee291 100644
--- a/chrome/browser/ui/views/payments/payment_request_sheet_controller.h
+++ b/chrome/browser/ui/views/payments/payment_request_sheet_controller.h
@@ -49,9 +49,19 @@ class PaymentRequestSheetController : public views::VectorIconButtonDelegate {
   // button should be displayed. The caller takes ownership of the button but
   // the view is guaranteed to be outlived by the controller so subclasses may
   // retain a raw pointer to the returned button (for example to control its
-  // enabled state).
+  // enabled state). See comment on CreatePaymentView for an illustration of the
+  // layout.
   virtual std::unique_ptr<views::Button> CreatePrimaryButton();
 
+  // Creates and returns the view to be displayed next to the "Pay" and "Cancel"
+  // buttons. May return an empty std::unique_ptr (nullptr) to indicate that no
+  // extra view is to be displayed.The caller takes ownership of the view but
+  // the view is guaranteed to be outlived by the controller so subclasses may
+  // retain a raw pointer to the returned view (for example to control its
+  // enabled state). See comment on CreatePaymentView for an illustration of the
+  // layout.
+  virtual std::unique_ptr<views::View> CreateExtraView();
+
   // views::VectorIconButtonDelegate:
   void ButtonPressed(views::Button* sender, const ui::Event& event) override;
 
@@ -68,7 +78,7 @@ class PaymentRequestSheetController : public views::VectorIconButtonDelegate {
   // |          CONTENT          |
   // |           VIEW            |
   // +---------------------------+
-  // |            | CANCEL | PAY | <-- footer
+  // | EXTRA VIEW | PAY | CANCEL | <-- footer
   // +---------------------------+
   std::unique_ptr<views::View> CreatePaymentView(
       std::unique_ptr<views::View> header_view,