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..780079b4acd6d37ffdbdae376a16fcd9e59d8e40 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/user_metrics_recorder_proxy.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_;
 
+  // Proxy's events from chrome/browser to ash::UserMetricsRecorder.

[Alexei Svitkine (slow), 2015/05/15 19:19:56]

Nit: Proxies.

But actually, I don't see this class logging any user actions. Is the intention
for this to come in future CLs? Otherwise, it doesn't seem to be accurately
named.

[bruthig, 2015/05/15 19:36:21]

Fixed Proxy's to Proxies.

Summary of discussion offline.  UserMetricsRecorderProxy intentionally uses
'Metrics' as opposed to 'Actions' to hopefully include histograms as well.  The
actual recording is done by the ash::UserMetricsRecorder class.

+  scoped_ptr<ash::UserMetricsRecorderProxy> user_metrics_recorder_proxy_;
+
 #if defined(OS_CHROMEOS)
   scoped_ptr<chromeos::DisplayConfigurationObserver>
       display_configuration_observer_;