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();
+
+// Enable browser test mode to make above functions test friendly.

[grt (UTC plus 2), 2016/05/27 16:26:28]

please explain what this does

[zmin, 2016/05/27 17:43:00]

Done.

+void EnableBrowserTest();

[grt (UTC plus 2), 2016/05/27 16:26:28]

call this something like SimulateDowngradeForTests to take advantage of the
"don't call test functions" presubmit check:
https://code.google.com/p/chromium/codesearch#chromium/src/PRESUBMIT.py&q=For...

[zmin, 2016/05/27 17:43:00]

Done.

+
+#endif  // CHROME_BROWSER_DOWNGRADE_USER_DATA_DOWNGRADE_H_