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

[Tom Sepez, 2016/11/02 20:23:22]

Parsing such an object in the browser is generally frowned upon; we use JSON in
the browser, but only when coming from a trusted source.

Do we need to process this in the browser or just pass it thru?  Presumably the
payment app can defend itself from malformed input?

[please use gerrit instead, 2016/11/02 20:35:17]

PaymentRequestImpl.java sanitizes and parses the string to JSONObject to
validate that the renderer is giving a valid JSON string to the browser. If
that's not the case, the browser tears down the Mojo pipe and closes the
payments UI.

AndroidPayPaymentApp.java and AndroidPayPaymentInstrument.java (not public)
process the JSONObject to determine the flags to use when calling into Android
Pay.

Once 3rd party payment apps are implemented, they will start receiving this as a
string. They will be responsible for protecting themselves.

+  // 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 {