[PaymentRequest] Replace '' with 'unknown' in PaymentComplete.

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

Index: third_party/WebKit/Source/modules/payments/PaymentResponse.idl
diff --git a/third_party/WebKit/Source/modules/payments/PaymentResponse.idl b/third_party/WebKit/Source/modules/payments/PaymentResponse.idl
index 686cf46f669702a06ba2139e9a888c4abf2cc3d0..71284189bf1531b1c9b8e6ddef88757c10ac2b73 100644
--- a/third_party/WebKit/Source/modules/payments/PaymentResponse.idl
+++ b/third_party/WebKit/Source/modules/payments/PaymentResponse.idl
@@ -7,7 +7,7 @@
 enum PaymentComplete {
     "success",
     "fail",
-    ""
+    "unknown"
 };
 
 // https://w3c.github.io/browser-payment-api/#paymentresponse-interface
@@ -25,5 +25,5 @@ enum PaymentComplete {
     readonly attribute PaymentAddress? shippingAddress;
     readonly attribute DOMString? shippingOption;
 
-    [CallWith=ScriptState] Promise<void> complete(optional PaymentComplete paymentResult = "");
+    [CallWith=ScriptState] Promise<void> complete(optional PaymentComplete paymentResult = "unknown");
 };