Use a stable device-user identifier for CryptAuth enrollments.

chrome/common/extensions/api/easy_unlock_private.idl

Index: chrome/common/extensions/api/easy_unlock_private.idl
diff --git a/chrome/common/extensions/api/easy_unlock_private.idl b/chrome/common/extensions/api/easy_unlock_private.idl
index 3e2ad141ef752b2f1406ea1df898285e9e97b48f..7d65a4736f6b7295544fe9768521b8ccd79929f4 100644
--- a/chrome/common/extensions/api/easy_unlock_private.idl
+++ b/chrome/common/extensions/api/easy_unlock_private.idl
@@ -144,6 +144,12 @@ namespace easyUnlockPrivate {
     // The user id.
     DOMString userId;
 
+    // A stable identifier for the user and device. If a user is removed and
+    // added to the same device, this id will remain the same. However, this id
+    // will be different if another user is added to the same device or if the
+    // same user is added on another device.
+    DOMString deviceUserId;
+
     // Whether the user is logged in. If not logged in, the app is running on
     // the signin screen.
     boolean loggedIn;