PaymentApp: Implement PaymentAppManager.getManifest().

components/payments/payment_app.mojom

Index: components/payments/payment_app.mojom
diff --git a/components/payments/payment_app.mojom b/components/payments/payment_app.mojom
index 59b92a1a99182a20f6b81eeceb6bef52db14d0eb..d4f16ee7fd290377ed40d4520d95f9e3db046a70 100644
--- a/components/payments/payment_app.mojom
+++ b/components/payments/payment_app.mojom
@@ -8,7 +8,7 @@ enum PaymentAppManifestError {
   NONE,
   NOT_IMPLEMENTED,
   NO_ACTIVE_WORKER,
-  STORE_MANIFEST_FAILED,
+  MANIFEST_STORAGE_OPERATION_FAILED,
 };
 
 struct PaymentAppOption {
@@ -27,4 +27,6 @@ struct PaymentAppManifest {
 interface PaymentAppManager {
   SetManifest(string service_worker_scope, PaymentAppManifest payment_app_manifest)
       => (PaymentAppManifestError error);
+  GetManifest(string service_worker_scope)
+      => (PaymentAppManifest payment_app_manifest, PaymentAppManifestError error);
 };