[cros] Use mocked URLFetcher and other mocks in ExistingUserController tests.

chrome/browser/chromeos/login/login_performer.cc

Index: chrome/browser/chromeos/login/login_performer.cc
diff --git a/chrome/browser/chromeos/login/login_performer.cc b/chrome/browser/chromeos/login/login_performer.cc
index ec814e909857996adc60cbb6b82cc10729516463..40c23b9972a42895f374e05bd587e19803fc7e8e 100644
--- a/chrome/browser/chromeos/login/login_performer.cc
+++ b/chrome/browser/chromeos/login/login_performer.cc
@@ -223,7 +223,10 @@ void LoginPerformer::OnPasswordChangeDetected() {
 
 void LoginPerformer::OnChecked(const std::string& username, bool success) {
   if (!delegate_) {
-    NOTREACHED();
+    // Delegate is reset in case of successful offline login.
+    // See ExistingUserConstoller::OnLoginSuccess().
+    // Case when user has changed password and enters old password
+    // does not block user from sign in yet.
     return;
   }
   delegate_->OnOnlineChecked(username, success);