[WebPayments] Implement state transitions in desktop WebPayments dialog.

chrome/app/generated_resources.grd

Index: chrome/app/generated_resources.grd
diff --git a/chrome/app/generated_resources.grd b/chrome/app/generated_resources.grd
index 3b5aa6ea4a5f212514f38e644c29f6646666c4bd..f9db138e91a1f71f10c8ee31081d77972a06ee72 100644
--- a/chrome/app/generated_resources.grd
+++ b/chrome/app/generated_resources.grd
@@ -15451,6 +15451,15 @@ read aloud to screenreader users to announce that a completion is available.">
       Enables granting and removing access to features through the Feature-Policy HTTP header.
     </message>
 
+    <!-- Payment Request -->
+    <message name="IDS_PAYMENT_REQUEST_PAYMENT_SHEET_TITLE" desc=" The title on top of the desktop Payment Request Payment Sheet screen">
+      Your payment
+    </message>
+
+    <message name="IDS_PAYMENT_REQUEST_ORDER_SUMMARY_TITLE" desc="The title on top of the desktop Payment Request Order Summary screen">
+      Order summary
+    </message>
+
    <!-- Safe Browsing Subresource Filter UI strings. -->
     <if expr="use_titlecase">
       <message name="IDS_FILTERED_DECEPTIVE_CONTENT_PROMPT_OK" desc="In Title Case: button which dismisses the Content blocked prompt without performing any actions.">