iframes with allowpaymentrequest attribute are allowed to make payment requests.

third_party/WebKit/Source/core/html/HTMLIFrameElement.idl

Index: third_party/WebKit/Source/core/html/HTMLIFrameElement.idl
diff --git a/third_party/WebKit/Source/core/html/HTMLIFrameElement.idl b/third_party/WebKit/Source/core/html/HTMLIFrameElement.idl
index 0a43b108ad1fd5e6c85bb2e2e9a51784fe94be08..e6b0e4b23d4eb61858828516ac8b864155accce9 100644
--- a/third_party/WebKit/Source/core/html/HTMLIFrameElement.idl
+++ b/third_party/WebKit/Source/core/html/HTMLIFrameElement.idl
@@ -27,6 +27,7 @@ interface HTMLIFrameElement : HTMLElement {
     [CEReactions, PutForwards=value] readonly attribute DOMTokenList sandbox;
     // Note: The seamless attribute was once supported, but was removed.
     [CEReactions, Reflect] attribute boolean allowFullscreen;
+    [CEReactions, Reflect] attribute boolean allowPaymentRequest;

[please use gerrit instead, 2016/10/13 00:26:48]

Instead of modifying the global iframe IDL here, you should try to define your
own partial interface file in modules/payments/HTMLIFrameElementPayments.idl
with the contents from the spec.

// https://w3c.github.io/browser-payment-api/#paymentrequest-and-iframes

[
    ImplementedAs=HTMLIFrameElementPayments,
] partial interface HTMLIFrameElement {
    [CEReactions, Reflect] attribute boolean allowPaymentRequest;
};

Then you can put the implementation in
modules/payments/HTMLIFrameElementPayments.h/cpp, or something like that.

See
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/timing/Wi...
and
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/timing/DO...
for an example.

[haraken, 2016/10/13 02:20:01]

Yes, this is a right way forward.

[pals, 2016/10/14 08:11:18]

Done.

     [CEReactions, Reflect] attribute DOMString width;
     [CEReactions, Reflect] attribute DOMString height;
     [CheckSecurity=ReturnValue] readonly attribute Document? contentDocument;