Only deactivate SupervisedUserService when removing its delegate.

chrome/browser/supervised_user/supervised_user_service.cc

Index: chrome/browser/supervised_user/supervised_user_service.cc
diff --git a/chrome/browser/supervised_user/supervised_user_service.cc b/chrome/browser/supervised_user/supervised_user_service.cc
index a5cae02584aa041052631496eb6c15b6e5630bdd..706997eb75e6a38c57c9fd1fad49f8b33c92b3cc 100644
--- a/chrome/browser/supervised_user/supervised_user_service.cc
+++ b/chrome/browser/supervised_user/supervised_user_service.cc
@@ -198,11 +198,14 @@ void SupervisedUserService::RegisterProfilePrefs(
 }
 
 void SupervisedUserService::SetDelegate(Delegate* delegate) {
-  if (delegate_ == delegate)
-    return;
-  // If the delegate changed, deactivate first to give the old delegate a chance
-  // to clean up.
-  SetActive(false);
+  if (delegate) {
+    // Changing delegates isn't allowed.
+    DCHECK(!delegate_);
+  } else {
+    // If the delegate is removed, deactivate first to give the old delegate a
+    // chance to clean up.
+    SetActive(false);
+  }
   delegate_ = delegate;
 }