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_
+
+// 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();
+
+// 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);

[sky, 2016/06/02 18:00:32]

Why do you need this? Can't you make the test harness setup the right
environment for you?

[zmin, 2016/06/02 18:21:18]

I use this for two things:
1) Avoid registry access. Unfortunately, RegistryOverrideManager is not working
for browser tests and editing the registry directly during the test is also not
a good idea for me.

2) Make sure the deletion task always got executed in the test. I can't find a
way to do it as it posted by PostTaskAfterStartup with BlockingPool. I have an
idea to make that API more test friendly but I need to talk with its
creator(michaeln@) when he back to the office from vacation.

+
+#endif  // CHROME_BROWSER_DOWNGRADE_USER_DATA_DOWNGRADE_H_