Reset user data directory and disk cache directory after downgrade.

chrome/browser/user_data_downgrade.h

Index: chrome/browser/user_data_downgrade.h
diff --git a/chrome/browser/user_data_downgrade.h b/chrome/browser/user_data_downgrade.h
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+// 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_USER_DATA_DOWNGRADE_H_
+#define CHROME_BROWSER_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();
+
+#endif  // CHROME_BROWSER_USER_DATA_DOWNGRADE_H_