Reset user data directory and disk cache directory after downgrade.

components/policy/resources/policy_templates.json

Index: components/policy/resources/policy_templates.json
diff --git a/components/policy/resources/policy_templates.json b/components/policy/resources/policy_templates.json
index edd7c26bbcd49a8ee7b930f6be40e1e03c92ee58..406dd847b692693d631a548eb0b927a2d8e8bc94 100644
--- a/components/policy/resources/policy_templates.json
+++ b/components/policy/resources/policy_templates.json
@@ -1415,7 +1415,9 @@
 
       See https://www.chromium.org/administrators/policy-list-3/user-data-directory-variables for a list of variables that can be used.
 
-      If this policy is left not set the default profile path will be used and the user will be able to override it with the '--user-data-dir' command line flag.''',
+      If this policy is left not set the default profile path will be used and the user will be able to override it with the '--user-data-dir' command line flag.
+
+      This policy must not be set to a volume's root directory, and should not be set to a directory used for other purposes.''',
       'label': '''Set user data directory''',
     },
     {
@@ -1437,7 +1439,9 @@
 
       See https://www.chromium.org/administrators/policy-list-3/user-data-directory-variables for a list of variables that can be used.
 
-      If this policy is left not set the default cache directory will be used and the user will be able to override it with the '--disk-cache-dir' command line flag.''',
+      If this policy is left not set the default cache directory will be used and the user will be able to override it with the '--disk-cache-dir' command line flag.
+
+      This policy must not be set to a volume's root directory, and should not be set to a directory used for other purposes.''',
       'label': '''Set disk cache directory''',
     },
     {