Added UMA statistics for changing the active window via click or touch events.

ash/metrics/desktop_task_switch_metric_recorder.h

Index: ash/metrics/desktop_task_switch_metric_recorder.h
diff --git a/ash/metrics/desktop_task_switch_metric_recorder.h b/ash/metrics/desktop_task_switch_metric_recorder.h
new file mode 100644
index 0000000000000000000000000000000000000000..67bbb00380e74af089a10c111e0f0c92f9f20c4a
--- /dev/null
+++ b/ash/metrics/desktop_task_switch_metric_recorder.h
@@ -0,0 +1,39 @@
+// 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 ASH_METRICS_DESKTOP_TASK_SWITCH_METRIC_RECORDER_H_
+#define ASH_METRICS_DESKTOP_TASK_SWITCH_METRIC_RECORDER_H_
+
+#include "ash/ash_export.h"
+#include "base/macros.h"
+#include "ui/wm/public/activation_change_observer.h"
+
+namespace ash {
+
+class UserMetricsRecorder;
+
+// Tracks metrics for task switches caused by the user activating a task window
+// by clicking or tapping on it.
+class ASH_EXPORT DesktopTaskSwitchMetricRecorder
+    : public aura::client::ActivationChangeObserver {
+ public:
+  explicit DesktopTaskSwitchMetricRecorder();
+  ~DesktopTaskSwitchMetricRecorder() override;
+
+  // aura::client::ActivationChangeObserver:
+  void OnWindowActivated(
+      aura::client::ActivationChangeObserver::ActivationReason reason,
+      aura::Window* gained_active,
+      aura::Window* lost_active) override;
+
+ private:
+  // Tracks the last active task window.
+  aura::Window* last_active_task_window_;
+
+  DISALLOW_COPY_AND_ASSIGN(DesktopTaskSwitchMetricRecorder);
+};
+
+}  // namespace ash
+
+#endif  // ASH_METRICS_DESKTOP_TASK_SWITCH_METRIC_RECORDER_H_