Mark PaymentDetails.error as experimental

There has not been a request to implement and ship this feature, so it
should be marked as experimental until such request has been sent out
and consequently approved.

BUG=648240

Review-Url: https://codereview.chromium.org/2530713002
Cr-Commit-Position: refs/heads/master@{#434664}

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

@@ -9,5 +9,5 @@ dictionary PaymentDetails {
     sequence<PaymentItem> displayItems;
     sequence<PaymentShippingOption> shippingOptions;
     sequence<PaymentDetailsModifier> modifiers;
-    DOMString error;
+    [RuntimeEnabled=PaymentDetailsError] DOMString error;
 };

third_party/WebKit/Source/platform/RuntimeEnabledFeatures.in

@@ -171,6 +171,7 @@ PassiveDocumentEventListeners status=stable
 PassiveEventListenersDueToFling status=experimental
 PassPaintVisualRectToCompositor
 PaymentApp status=experimental
+PaymentDetailsError status=experimental
 // PaymentRequest is enabled by default on Android
 PaymentRequest status=experimental
 PaymentRequestIFrame status=experimental