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

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

Index: chrome/browser/ui/ash/metrics/user_metrics_recorder_proxy_impl.h
diff --git a/chrome/browser/ui/ash/metrics/user_metrics_recorder_proxy_impl.h b/chrome/browser/ui/ash/metrics/user_metrics_recorder_proxy_impl.h
new file mode 100644
index 0000000000000000000000000000000000000000..dd81f6d3fd0e4c7de148d3188469b3592ae7cb1f
--- /dev/null
+++ b/chrome/browser/ui/ash/metrics/user_metrics_recorder_proxy_impl.h
@@ -0,0 +1,26 @@
+// 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_IMPL_H_
+#define CHROME_BROWSER_UI_ASH_METRICS_USER_METRICS_RECORDER_PROXY_IMPL_H_
+
+#include <string>
+
+#include "ash/metrics/user_metrics_recorder_proxy.h"
+#include "base/macros.h"
+
+class UserMetricsRecorderProxyImpl : public ash::UserMetricsRecorderProxy {
+ public:
+  enum TaskSwitchSource { kTabStrip };
+
+  UserMetricsRecorderProxyImpl();
+  ~UserMetricsRecorderProxyImpl() override;
+
+  void OnTaskSwitch(TaskSwitchSource task_switch_source);
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(UserMetricsRecorderProxyImpl);
+};
+
+#endif  // CHROME_BROWSER_UI_ASH_METRICS_USER_METRICS_RECORDER_PROXY_IMPL_H_