Add about flag for offline page experiment

components/offline_pages/offline_page_switches.h

Index: components/offline_pages/offline_page_switches.h
diff --git a/components/offline_pages/offline_page_switches.h b/components/offline_pages/offline_page_switches.h
new file mode 100644
index 0000000000000000000000000000000000000000..4df0618ff005830423dc76803feccd43df02e6ac
--- /dev/null
+++ b/components/offline_pages/offline_page_switches.h
@@ -0,0 +1,15 @@
+// Copyright 2015 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.
+
+#ifndef COMPONENTS_OFFLINE_PAGES_OFFLINE_PAGES_SWITCHES_H_
+#define COMPONENTS_OFFLINE_PAGES_OFFLINE_PAGES_SWITCHES_H_
+
+namespace switches {
+
+extern const char kEnableOfflinePages[];
+extern const char kDisableOfflinePages[];
+
+}  // namespace switches
+
+#endif  // COMPONENTS_OFFLINE_PAGES_OFFLINE_PAGES_SWITCHES_H_