Reset user data directory and disk cache directory after downgrade.

chrome/browser/downgrade/user_data_downgrade.h

Index: chrome/browser/downgrade/user_data_downgrade.h
diff --git a/chrome/browser/downgrade/user_data_downgrade.h b/chrome/browser/downgrade/user_data_downgrade.h
new file mode 100644
index 0000000000000000000000000000000000000000..f7d7dbca9a39f1d87c0e9994fefde3528bca2133
--- /dev/null
+++ b/chrome/browser/downgrade/user_data_downgrade.h
@@ -0,0 +1,21 @@
+// 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();

[sky, 2016/05/24 16:54:44]

Does the uninstaller have the option of removing profile data? If so, is it
possible to use that code?

[zmin, 2016/05/24 17:32:52]

Yes, the uninstaller has that option. However, the code can not be reused easily
because of some reasons:

1) This function will not delete the profile data directly as uninstaller but
all directories renamed by previous function. It's also responsible for
searching all renamed directories.

2) The uninstaller removing does not support group policy or command line flag
now.

3) This function doesn't need MOVEFILE_DELAY_UNTIL_REBOOT as uninstaller. If the
profiles can not be deleted, we can simple retry in the next launch.

+
+#endif  // CHROME_BROWSER_DOWNGRADE_USER_DATA_DOWNGRADE_H_