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

chrome/browser/ui/ash/chrome_shell_delegate_chromeos.cc

Index: chrome/browser/ui/ash/chrome_shell_delegate_chromeos.cc
diff --git a/chrome/browser/ui/ash/chrome_shell_delegate_chromeos.cc b/chrome/browser/ui/ash/chrome_shell_delegate_chromeos.cc
index 1b617147813255fcd5ccb26f3f04ae4c346c3996..847cf7b28d55e189e9ae7c33b974c19090ba5e4a 100644
--- a/chrome/browser/ui/ash/chrome_shell_delegate_chromeos.cc
+++ b/chrome/browser/ui/ash/chrome_shell_delegate_chromeos.cc
@@ -217,10 +217,13 @@ void ChromeShellDelegate::PreInit() {
   // 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() {