Reset user data directory and disk cache directory after downgrade.

chrome/browser/downgrade/user_data_downgrade.h

+// Copyright 2016 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 CHROME_BROWSER_DOWNGRADE_USER_DATA_DOWNGRADE_H_
+#define CHROME_BROWSER_DOWNGRADE_USER_DATA_DOWNGRADE_H_
+
+#include "base/files/file_path.h"
+#include "base/version.h"
+
+// The suffix of pending deleted directory.
+extern const base::FilePath::CharType g_downgrade_delete_suffix[];

[grt (UTC plus 2), 2016/06/07 01:25:28]

these are constants, so use kFoo naming

[zmin, 2016/06/07 14:41:49]

Done.

+// The name of "Browser Version" file.
+extern const base::FilePath::CharType g_downgrade_browser_version_file[];
+
+// Moves aside all directories containing user data to prevent the browser from
+// reading them when Chrome is downgraded from a higher version. When called
+// early in startup, this will result in the browser performing first-run for
+// the user. To avoid long-running I/O operations, this function merely moves
+// directories aside. A subsequent call to DeleteMovedUserDataSoon (below) must
+// be made later in startup to free up disk space.
+void MoveUserDataForFirstRunAfterDowngrade();
+
+// Update the content of "Browser Version" file with current version number
+// in |user_data_dir|.
+void UpdateBrowserVersion(const base::FilePath& user_data_dir);
+
+// Get the content of "Browser Version" file in |user_data_dir|.
+base::Version GetBrowserVersion(const base::FilePath& user_data_dir);
+
+// Schedules a search for the removal of any directories moved aside by
+// MoveUserDataForFirstRunAfterDowngrade. This operation is idempotent,
+// and may be safely called when no such directories exist.
+void DeleteMovedUserDataSoon();
+
+// Changes the behavior of the above functions to simulate a downgrade for use
+// in tests. MoveUserDataForFirstRunAfterDowngrade will no longer rely on
+// registry and DeleteMovedUserDataSoon will always post task into blocking
+// pool for brower test.
+void SimulateDowngradeForTest(bool is_browser_test);
+
+#endif  // CHROME_BROWSER_DOWNGRADE_USER_DATA_DOWNGRADE_H_