Add data parameter to payment details modifier.

components/payments/payment_request.mojom

Index: components/payments/payment_request.mojom
diff --git a/components/payments/payment_request.mojom b/components/payments/payment_request.mojom
index dc9ab3374af80aed7ef648580855df5aa42f40e3..cf27fb491c09988a81e053411c865fecd93dd27f 100644
--- a/components/payments/payment_request.mojom
+++ b/components/payments/payment_request.mojom
@@ -97,6 +97,16 @@ struct PaymentDetailsModifier {
   array<string> supported_methods;
   PaymentItem? total;
   array<PaymentItem> additional_display_items;
+  // A JSON string built by the renderer from a JavaScript object that the
+  // merchant website provides. The renderer uses
+  // blink::JSONObject::toJSONString() to generate this string. The browser
+  // parses the string via JSONObject(JsonSanitizer.sanitize(stringified_data))
+  // and passes a part of the JSON object to the payment app, for example
+  // Android Pay. There's no one format for this object, so richer types cannot
+  // be used. A simple example:
+  //
+  // {"gateway": "stripe"}
+  string stringified_data;
 };
 
 struct PaymentDetails {