PaymentApp: Implement PaymentAppProvider for LayoutTest.

content/shell/BUILD.gn

Index: content/shell/BUILD.gn
diff --git a/content/shell/BUILD.gn b/content/shell/BUILD.gn
index 6749acd82ed5b109af1f277028acb934736509b1..d85d4b23cad87061b58474729ba5d755714a8bcb 100644
--- a/content/shell/BUILD.gn
+++ b/content/shell/BUILD.gn
@@ -84,6 +84,8 @@ static_library("content_shell_lib") {
     "browser/layout_test/layout_test_message_filter.h",
     "browser/layout_test/layout_test_notification_manager.cc",
     "browser/layout_test/layout_test_notification_manager.h",
+    "browser/layout_test/layout_test_payment_app_provider_impl.cc",
+    "browser/layout_test/layout_test_payment_app_provider_impl.h",
     "browser/layout_test/layout_test_permission_manager.cc",
     "browser/layout_test/layout_test_permission_manager.h",
     "browser/layout_test/layout_test_push_messaging_service.cc",
@@ -761,5 +763,6 @@ if (is_mac) {
 mojom("mojo_bindings") {
   sources = [
     "common/layout_test/layout_test_bluetooth_fake_adapter_setter.mojom",
+    "common/layout_test/layout_test_payment_app_provider.mojom",
   ]
 }