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

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

Index: chrome/browser/ui/ash/metrics/user_metrics_recorder_proxy.cc
diff --git a/chrome/browser/ui/ash/metrics/user_metrics_recorder_proxy.cc b/chrome/browser/ui/ash/metrics/user_metrics_recorder_proxy.cc
new file mode 100644
index 0000000000000000000000000000000000000000..69bb08962fdd7088e48ab13874950040a5e5d4a2
--- /dev/null
+++ b/chrome/browser/ui/ash/metrics/user_metrics_recorder_proxy.cc
@@ -0,0 +1,51 @@
+// 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/user_metrics_recorder_proxy.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/tabs/tab_strip_model.h"
+
+namespace ash {
+
+UserMetricsRecorderProxy::UserMetricsRecorderProxy() {
+  BrowserList::AddObserver(this);
+}
+
+UserMetricsRecorderProxy::~UserMetricsRecorderProxy() {
+  BrowserList::RemoveObserver(this);
+}
+
+void UserMetricsRecorderProxy::OnBrowserAdded(Browser* browser) {
+  browser->tab_strip_model()->AddObserver(this);
+}
+
+void UserMetricsRecorderProxy::OnBrowserRemoved(Browser* browser) {
+  browser->tab_strip_model()->RemoveObserver(this);
+}
+
+void UserMetricsRecorderProxy::ActiveTabChanged(
+    content::WebContents* old_contents,
+    content::WebContents* new_contents,
+    int index,
+    int reason) {
+  if (reason & CHANGE_REASON_USER_GESTURE) {
+    OnTabSwitchedByUserGesture();
+  }
+}
+
+void UserMetricsRecorderProxy::OnTabSwitchedByUserGesture() {
+  CHECK(ash::Shell::HasInstance());
+  ash::Shell::GetInstance()
+      ->metrics()
+      ->task_switch_metrics_recorder()
+      .OnTaskSwitch(ash::TaskSwitchMetricsRecorder::kTabStrip);
+}
+
+}  // namespace ash