Contains a regular expression which is used to determine which Google accounts can be set as browser primary accounts in Google Chrome (i.e. the account that is chosen during the Sync opt-in flow).
An appropriate error is displayed if a user tries to set a browser primary account with a username that does not match this pattern.
If this policy is left not set or blank, then the user can set any Google account as a browser primary account in Google Chrome.
Example value: .*@example.com
|Registry Hive||HKEY_LOCAL_MACHINE or HKEY_CURRENT_USER|