PaymentApp: Implement GetAllManifests() in PaymentAppContext.

content/test/BUILD.gn

Index: content/test/BUILD.gn
diff --git a/content/test/BUILD.gn b/content/test/BUILD.gn
index 7622839031c3315bbe5808b3d0547cc41fb3f3a5..c46efd4abc24f44462bfc70947ed3ff8bd0f087e 100644
--- a/content/test/BUILD.gn
+++ b/content/test/BUILD.gn
@@ -1140,6 +1140,9 @@ test("content_unittests") {
     "../browser/notifications/notification_id_generator_unittest.cc",
     "../browser/notifications/platform_notification_context_unittest.cc",
     "../browser/notifications/type_converters_unittest.cc",
+    "../browser/payments/payment_app_content_unittest_base.cc",
+    "../browser/payments/payment_app_content_unittest_base.h",
+    "../browser/payments/payment_app_context_impl_unittest.cc",
     "../browser/payments/payment_app_manager_unittest.cc",
     "../browser/presentation/presentation_service_impl_unittest.cc",
     "../browser/presentation/presentation_type_converters_unittest.cc",