Added UMA metrics for ChromeOS first-run UI.

chrome/browser/chromeos/first_run/step_names.cc

 // Copyright 2013 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/first_run/step_names.h"
 
 namespace chromeos {
 namespace first_run {
 
+// This values should be synced with ids of corresponding steps in HTML-side.
+// Also there are metric recording how much time user spent on every step.
+// Metric's name has format "CrosFirstRun.TimeSpentOn[StepName]Step", where
+// |StepName| is camel-cased version of |step-name|. Corresponding record
+// should be added to "histograms.xml" file for every step listed here.
 const char kAppListStep[] = "app-list";
 const char kTrayStep[] = "tray";
 const char kHelpStep[] = "help";
 
 }  // namespace first_run
 }  // namespace chromeos