app_shell: Add experimental shellPrivate extension API

extensions/browser/api/shell_private/shell_private_api.cc

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "extensions/browser/api/shell_private/shell_private_api.h"
+
+namespace extensions {
+
+ShellPrivatePrintHelloFunction::ShellPrivatePrintHelloFunction() {
+}
+
+ShellPrivatePrintHelloFunction::~ShellPrivatePrintHelloFunction() {
+}
+
+ExtensionFunction::ResponseAction ShellPrivatePrintHelloFunction::Run() {
+  // Warning so it shows in Release and avoids presubmit check for INFO.
+  LOG(WARNING) << "hello";
+  return RespondNow(NoArguments());
+}
+
+}  // namespace extensions