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

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

Index: chrome/browser/ui/ash/metrics/chrome_user_metrics_recorder.cc
diff --git a/chrome/browser/ui/ash/metrics/chrome_user_metrics_recorder.cc b/chrome/browser/ui/ash/metrics/chrome_user_metrics_recorder.cc
new file mode 100644
index 0000000000000000000000000000000000000000..81e91083cbc13e27d12af153b89accff8b0bce05
--- /dev/null
+++ b/chrome/browser/ui/ash/metrics/chrome_user_metrics_recorder.cc
@@ -0,0 +1,47 @@
+// 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.
+
+#include "chrome/browser/ui/ash/metrics/chrome_user_metrics_recorder.h"
+
+#include "ash/metrics/task_switch_metrics_recorder.h"
+#include "ash/metrics/user_metrics_recorder.h"
+#include "ash/shell.h"
+#include "base/logging.h"
+#include "chrome/browser/ui/browser.h"
+#include "chrome/browser/ui/browser_list.h"
+#include "chrome/browser/ui/host_desktop.h"
+#include "chrome/browser/ui/tabs/tab_strip_model.h"
+
+ChromeUserMetricsRecorder::ChromeUserMetricsRecorder() {
+  BrowserList::AddObserver(this);
+}
+
+ChromeUserMetricsRecorder::~ChromeUserMetricsRecorder() {
+  DCHECK(BrowserList::GetInstance(chrome::HOST_DESKTOP_TYPE_ASH)->empty());
+  BrowserList::RemoveObserver(this);
+}
+
+void ChromeUserMetricsRecorder::OnBrowserAdded(Browser* browser) {
+  browser->tab_strip_model()->AddObserver(this);
+}
+
+void ChromeUserMetricsRecorder::OnBrowserRemoved(Browser* browser) {
+  browser->tab_strip_model()->RemoveObserver(this);
+}
+
+void ChromeUserMetricsRecorder::ActiveTabChanged(
+    content::WebContents* old_contents,
+    content::WebContents* new_contents,
+    int index,
+    int reason) {
+  if (reason & CHANGE_REASON_USER_GESTURE)
+    OnTabSwitchedByUserGesture();
+}
+
+void ChromeUserMetricsRecorder::OnTabSwitchedByUserGesture() {
+  ash::Shell::GetInstance()
+      ->metrics()
+      ->task_switch_metrics_recorder()
+      .OnTaskSwitch(ash::TaskSwitchMetricsRecorder::kTabStrip);
+}