Added the Ash.Tab.TimeBetweenSwitchToExistingTabUserActions histogram to track time between tab swit

chrome/browser/ui/ash/chrome_shell_delegate_chromeos.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/ui/ash/chrome_shell_delegate.h"
 
 #include "ash/accelerators/magnifier_key_scroller.h"
 #include "ash/accelerators/spoken_feedback_toggler.h"
 #include "ash/accessibility_delegate.h"
 #include "ash/wm/mru_window_tracker.h"
@@ skipping 199 matching lines @@
   return base::CommandLine::ForCurrentProcess()->HasSwitch(
       chromeos::switches::kFirstExecAfterBoot);
 }
 
 void ChromeShellDelegate::PreInit() {
   chromeos::LoadDisplayPreferences(IsFirstRunAfterBoot());
   // Set the observer now so that we can save the initial state
   // in Shell::Init.
   display_configuration_observer_.reset(
       new chromeos::DisplayConfigurationObserver());
+
+  user_metrics_recorder_proxy_.reset(new ash::UserMetricsRecorderProxy);
 }
 
 void ChromeShellDelegate::PreShutdown() {
   display_configuration_observer_.reset();
+  user_metrics_recorder_proxy_.reset();

[bruthig, 2015/05/15 19:11:27]

oshima@, is it safe to assume that all Browser's have called
UserMetricsRecorderProxy::OnBrowserRemoved(...) at this point?  Or does the
UserMetricsRecorderProxy class need to track which Browser's it has attached to
and disconnect the observer during ~UserMetricsRecorderProxy()?

 }
 
 ash::SessionStateDelegate* ChromeShellDelegate::CreateSessionStateDelegate() {
   return new SessionStateDelegateChromeos;
 }
 
 ash::AccessibilityDelegate* ChromeShellDelegate::CreateAccessibilityDelegate() {
   return new AccessibilityDelegateImpl;
 }
 
@@ skipping 41 matching lines @@
 }
 
 void ChromeShellDelegate::PlatformInit() {
   registrar_.Add(this,
                  chrome::NOTIFICATION_LOGIN_USER_PROFILE_PREPARED,
                  content::NotificationService::AllSources());
   registrar_.Add(this,
                  chrome::NOTIFICATION_SESSION_STARTED,
                  content::NotificationService::AllSources());
 }