[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 d7c73e7f60f20b8224ca6e4684f44c2af80d1eeb..fbe3339ddb1ee6ac31ee8e5189301bd12d6fc9b3 100644
--- a/chrome/browser/ui/views/payments/payment_request_sheet_controller.h
+++ b/chrome/browser/ui/views/payments/payment_request_sheet_controller.h
@@ -55,6 +55,17 @@ class PaymentRequestSheetController : public views::VectorIconButtonDelegate {
   // enabled state).
   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).
+  // +---------------------------+
+  // | EXTRA VIEW | PAY | CANCEL |
+  // +---------------------------+
+  virtual std::unique_ptr<views::View> CreateExtraView();

[Mathieu, 2017/02/23 00:39:07]

We should keep one of CreateExtraView or CreateLeadingFooterView

+
   // views::VectorIconButtonDelegate:
   void ButtonPressed(views::Button* sender, const ui::Event& event) override;
 
@@ -71,7 +82,7 @@ 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,