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

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

Index: chrome/browser/ui/ash/metrics/user_metrics_recorder_proxy_impl.cc
diff --git a/chrome/browser/ui/ash/metrics/user_metrics_recorder_proxy_impl.cc b/chrome/browser/ui/ash/metrics/user_metrics_recorder_proxy_impl.cc
new file mode 100644
index 0000000000000000000000000000000000000000..cacdd8754cb5eec0160be83db880870515866b54
--- /dev/null
+++ b/chrome/browser/ui/ash/metrics/user_metrics_recorder_proxy_impl.cc
@@ -0,0 +1,27 @@
+// 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_impl.h"
+
+#include "ash/metrics/task_switch_metrics_recorder.h"
+#include "ash/metrics/user_metrics_recorder.h"
+#include "ash/shell.h"
+
+UserMetricsRecorderProxyImpl::UserMetricsRecorderProxyImpl() {
+}
+
+UserMetricsRecorderProxyImpl::~UserMetricsRecorderProxyImpl() {
+}
+
+void UserMetricsRecorderProxyImpl::OnTaskSwitch(
+    TaskSwitchSource task_switch_source) {
+  switch (task_switch_source) {
+    case UserMetricsRecorderProxyImpl::kTabStrip:
+      ash::Shell::GetInstance()
+          ->metrics()
+          ->task_switch_metrics_recorder()
+          .OnTaskSwitch(ash::TaskSwitchMetricsRecorder::kTabStrip);
+      break;
+  }
+}