Replace sequence with FrozenArray in PaymentRequestEvent.idl.

According to the spec[1], |methodData| and |modifiers| attributes should be
FrozenArray instead of sequence.

[1] https://w3c.github.io/payment-handler/#the-paymentrequestevent

BUG=661608,720027

Review-Url: https://codereview.chromium.org/2934703002
Cr-Commit-Position: refs/heads/master@{#478647}

third_party/WebKit/Source/modules/payments/PaymentRequestEvent.idl

@@ -12,9 +12,9 @@
     readonly attribute USVString topLevelOrigin;
     readonly attribute USVString paymentRequestOrigin;
     readonly attribute DOMString paymentRequestId;
-    readonly attribute sequence<PaymentMethodData> methodData;
+    readonly attribute FrozenArray<PaymentMethodData> methodData;
     readonly attribute PaymentItem total;
-    readonly attribute sequence<PaymentDetailsModifier> modifiers;
+    readonly attribute FrozenArray<PaymentDetailsModifier> modifiers;
     readonly attribute DOMString instrumentKey;
 
     [CallWith=ScriptState] Promise<WindowClient> openWindow(USVString url);