Add Login.PromptToLoginTime UMA metric

chrome/browser/chromeos/login/existing_user_controller.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/chromeos/login/existing_user_controller.h"
 
 #include <memory>
 #include <vector>
 
 #include "base/bind.h"
@@ skipping 499 matching lines @@
     // sure that the user's cryptohome is encrypted in ext4 dircrypto to run the
     // latest Android runtime.
     UserContext new_user_context = user_context;
     new_user_context.SetIsForcingDircrypto(
         ShouldForceDircrypto(new_user_context.GetAccountId()));
     login_performer_->PerformLogin(new_user_context, auth_mode);
     RecordPasswordLoginEvent(new_user_context);
   }
   SendAccessibilityAlert(
       l10n_util::GetStringUTF8(IDS_CHROMEOS_ACC_LOGIN_SIGNING_IN));
+  if (!time_init_.is_null()) {
+    base::TimeDelta delta = base::Time::Now() - time_init_;
+    UMA_HISTOGRAM_MEDIUM_TIMES("Login.PromptToLoginTime", delta);
+    time_init_ = base::Time();  // Reset to null.
+  }
 }
 
 void ExistingUserController::ContinuePerformLogin(
     LoginPerformer::AuthorizationMode auth_mode,
     const UserContext& user_context) {
   login_performer_->PerformLogin(user_context, auth_mode);
 }
 
 void ExistingUserController::MigrateUserData(const std::string& old_password) {
   // LoginPerformer instance has state of the user so it should exist.
@@ skipping 963 matching lines @@
   login_display_->ShowUnrecoverableCrypthomeErrorDialog();
 }
 
 void ExistingUserController::ClearRecordedNames() {
   display_email_.clear();
   display_name_.clear();
   given_name_.clear();
 }
 
 }  // namespace chromeos