Implement a policy to autologin a public account.

chrome/browser/chromeos/policy/proto/chrome_device_policy.proto

Index: chrome/browser/chromeos/policy/proto/chrome_device_policy.proto
diff --git a/chrome/browser/chromeos/policy/proto/chrome_device_policy.proto b/chrome/browser/chromeos/policy/proto/chrome_device_policy.proto
index d317d29a0f46f54e480e4dcdaab22eb4b33c9753..76e692c7ab2fffa125142daea3581ab23d76b482 100644
--- a/chrome/browser/chromeos/policy/proto/chrome_device_policy.proto
+++ b/chrome/browser/chromeos/policy/proto/chrome_device_policy.proto
@@ -231,6 +231,15 @@ message DeviceLocalAccountsProto {
   // The list of device-local accounts (i.e. accounts without an associated
   // cloud-backed profile) that are available on the device.
   repeated DeviceLocalAccountInfoProto account = 1;
+
+  // The identifier of the device-local account to which the device
+  // should be logged in automatically.  Should be equal to one of the
+  // ids in DeviceLocalAccountInfoProto.
+  optional string auto_login_id = 2;
+
+  // The amount of time, in milliseconds, that should elapse at the signin
+  // screen without user interaction before automatically logging in.
+  optional int64 auto_login_delay = 3;
 }
 
 message AllowRedeemChromeOsRegistrationOffersProto {