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

chrome/browser/ui/ash/chrome_shell_delegate.h

Index: chrome/browser/ui/ash/chrome_shell_delegate.h
diff --git a/chrome/browser/ui/ash/chrome_shell_delegate.h b/chrome/browser/ui/ash/chrome_shell_delegate.h
index a467a6ee8c1672fab697f2f93a7a2e41750c53ad..ae87ac1304ec80eabde11662cb4f344843cd6465 100644
--- a/chrome/browser/ui/ash/chrome_shell_delegate.h
+++ b/chrome/browser/ui/ash/chrome_shell_delegate.h
@@ -13,6 +13,7 @@
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/observer_list.h"
+#include "chrome/browser/ui/ash/metrics/chrome_user_metrics_recorder.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
 
@@ -93,6 +94,9 @@ class ChromeShellDelegate : public ash::ShellDelegate,
 
   ObserverList<ash::VirtualKeyboardStateObserver> keyboard_state_observer_list_;
 
+  // Proxies events from chrome/browser to ash::UserMetricsRecorder.
+  scoped_ptr<ChromeUserMetricsRecorder> chrome_user_metrics_recorder_;
+
 #if defined(OS_CHROMEOS)
   scoped_ptr<chromeos::DisplayConfigurationObserver>
       display_configuration_observer_;