PaymentHandler: Implement PaymentInstruments.keys().

components/payments/mojom/payment_app.mojom

Index: components/payments/mojom/payment_app.mojom
diff --git a/components/payments/mojom/payment_app.mojom b/components/payments/mojom/payment_app.mojom
index 702e287629cea798777b4dffb343ea768f180998..1b9ee5c7b80a416501556f5ad6d9872f9f897fd1 100644
--- a/components/payments/mojom/payment_app.mojom
+++ b/components/payments/mojom/payment_app.mojom
@@ -50,12 +50,14 @@ interface PaymentManager {
       => (PaymentAppManifest payment_app_manifest, PaymentAppManifestError error);
   DeletePaymentInstrument(string instrument_key)
       => (PaymentHandlerStatus status);
-  SetPaymentInstrument(string instrument_key, PaymentInstrument instrument)
-      => (PaymentHandlerStatus status);
-  HasPaymentInstrument(string instrument_key)
-      => (PaymentHandlerStatus status);
   GetPaymentInstrument(string instrument_key)
       => (PaymentInstrument instrument, PaymentHandlerStatus status);
+  KeysOfPaymentInstruments()
+      => (array<string> keys, PaymentHandlerStatus status);
+  HasPaymentInstrument(string instrument_key)
+      => (PaymentHandlerStatus status);
+  SetPaymentInstrument(string instrument_key, PaymentInstrument instrument)
+      => (PaymentHandlerStatus status);
 };
 
 struct PaymentAppRequest {