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

chrome/browser/ui/ash/metrics/user_metrics_recorder_proxy.h

Index: chrome/browser/ui/ash/metrics/user_metrics_recorder_proxy.h
diff --git a/chrome/browser/ui/ash/metrics/user_metrics_recorder_proxy.h b/chrome/browser/ui/ash/metrics/user_metrics_recorder_proxy.h
new file mode 100644
index 0000000000000000000000000000000000000000..32bc89b3ccab1b88c9f66ccb278e7f33d1e06e82
--- /dev/null
+++ b/chrome/browser/ui/ash/metrics/user_metrics_recorder_proxy.h
@@ -0,0 +1,46 @@
+// Copyright 2015 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.
+
+#ifndef CHROME_BROWSER_UI_ASH_METRICS_USER_METRICS_RECORDER_PROXY_H_
+#define CHROME_BROWSER_UI_ASH_METRICS_USER_METRICS_RECORDER_PROXY_H_
+
+#include <string>

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

Is this include needed?

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

Removed.

+
+#include "base/macros.h"
+#include "chrome/browser/ui/browser_list_observer.h"
+#include "chrome/browser/ui/tabs/tab_strip_model_observer.h"
+
+namespace content {
+class WebContents;
+}  // namespace content
+
+namespace ash {
+
+// A bridge proxy between chrome/browser events and ash::UserMetricsRecorder.
+class UserMetricsRecorderProxy : public chrome::BrowserListObserver,
+                                 public TabStripModelObserver {
+ public:
+  UserMetricsRecorderProxy();
+  ~UserMetricsRecorderProxy() override;
+
+  // chrome::BroswerListObserver:
+  void OnBrowserAdded(Browser* browser) override;
+  void OnBrowserRemoved(Browser* browser) override;
+
+  // TabStripModelObserver:
+  void ActiveTabChanged(content::WebContents* old_contents,
+                        content::WebContents* new_contents,
+                        int index,
+                        int reason) override;
+
+ private:
+  // Called when a new tab becomes active due to a user gesture.

[tdanderson, 2015/05/15 19:23:09]

nit: using the term "new tab" could be confusing since we're not measuring
opening new tabs as task switches.

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

Done.

+  void OnTabSwitchedByUserGesture();
+
+  DISALLOW_COPY_AND_ASSIGN(UserMetricsRecorderProxy);
+};
+
+}  // namespace ash
+
+#endif  // CHROME_BROWSER_UI_ASH_METRICS_USER_METRICS_RECORDER_PROXY_H_