Replace own callback handling with Promises.

chrome/app/generated_resources.grd

Index: chrome/app/generated_resources.grd
diff --git a/chrome/app/generated_resources.grd b/chrome/app/generated_resources.grd
index c73a3af03a21d5f0112aa41d4666be02f62b30fd..41f8dbdec510c2f93657b6667c318151ed38412a 100644
--- a/chrome/app/generated_resources.grd
+++ b/chrome/app/generated_resources.grd
@@ -14711,6 +14711,9 @@ Please check your email at <ph name="ACCOUNT_EMAIL">$2<ex>jane.doe@gmail.com</ex
     <message name="IDS_MANAGED_USER_ALREADY_ON_THIS_DEVICE" desc="Text shown in the list of supervised users beside the name of a supervised user that already exists on the current device.">
       (already on this device)
     </message>
+    <message name="IDS_MANAGED_USER_NAME_CONFLICT" desc="Text shown in the list of supervised users beside the name of a supervised user that has the same name as a profile on this device.">
+      (name already in use)
+    </message>
     <message name="IDS_MANAGED_USER_NO_EXISTING_ERROR" desc="Message shown to the custodian when he opens the 'supervised user import' overlay but he has no existing supervised users to import.">
       Oops! Looks like you have no existing supervised users to import. Please create one or more from another device then you can import them here.
     </message>