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

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

Index: chrome/browser/ui/ash/metrics/chrome_user_metrics_recorder.h
diff --git a/chrome/browser/ui/ash/metrics/chrome_user_metrics_recorder.h b/chrome/browser/ui/ash/metrics/chrome_user_metrics_recorder.h
new file mode 100644
index 0000000000000000000000000000000000000000..81236e99012d8acffd8ba4f8ab3453fd0feb8c6e
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+++ b/chrome/browser/ui/ash/metrics/chrome_user_metrics_recorder.h
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+// 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_CHROME_USER_METRICS_RECORDER_H_
+#define CHROME_BROWSER_UI_ASH_METRICS_CHROME_USER_METRICS_RECORDER_H_
+
+#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
+
+// A bridge proxy between chrome/browser events and ash::UserMetricsRecorder.
+class ChromeUserMetricsRecorder : public chrome::BrowserListObserver,
+                                  public TabStripModelObserver {
+ public:
+  ChromeUserMetricsRecorder();
+  ~ChromeUserMetricsRecorder() 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 different tab becomes active due to a user gesture.
+  void OnTabSwitchedByUserGesture();
+
+  DISALLOW_COPY_AND_ASSIGN(ChromeUserMetricsRecorder);
+};
+
+#endif  // CHROME_BROWSER_UI_ASH_METRICS_CHROME_USER_METRICS_RECORDER_H_