UIProxyConfigService is currently used:
  1. by networking_private_api to calculate the effective proxy settings
     for a network - more specifically, to override user proxy settings
     defined in shill with proxy config from prefs (which could be set 
     either by policy or an extension)
  2. by system tray to display the "Network might be monitored" warning
     when a proxy is used

The active UIProxyConfigService is recreated  with new user prefs on each user login (by network_pref_state_observer.cc, which observes NOTIFICATION_LOGIN_USER_PROFILE_PREPARED to determine when the config service should be recreated). This means that UIProxyConfigService will use prefs from the last logged in user, which is not the desired behavior in either case:

For network_private_api, generally the primary user's prefs should be used (though, there's no need to use the cached prefs pointer in this case - networking_private_api can get the appropriate pref service from the extension function context, and pass it to the UIProxyConfigService).

For system tray, the active user prefs would be preferred (given that the proxy setting can differ for primary and secondary users).
Currently, when used by system tray, the user prefs are ignored all together - see https://crbug.com/919691