app_shell: Add experimental shellPrivate extension API

extensions/common/api/shell_private.idl

Index: extensions/common/api/shell_private.idl
diff --git a/extensions/common/api/shell_private.idl b/extensions/common/api/shell_private.idl
new file mode 100644
index 0000000000000000000000000000000000000000..f964c29eced1aca1e689d3eb164c5a2dcbd8051a
--- /dev/null
+++ b/extensions/common/api/shell_private.idl
@@ -0,0 +1,12 @@
+// 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.
+
+// <code>chrome.shellPrivate</code> API with experimental APIs for app_shell.
+[nodoc] namespace shellPrivate {
+
+  interface Functions {
+    // Prints "hello" to the Chrome log.
+    static void printHello();
+  };
+};