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

ash/metrics/desktop_task_switch_metric_recorder.cc

+// 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 "ash/metrics/desktop_task_switch_metric_recorder.h"
+
+#include "ash/metrics/user_metrics_recorder.h"
+#include "ash/shell.h"
+#include "ash/wm/window_util.h"
+#include "ui/wm/public/activation_client.h"
+
+namespace ash {
+
+DesktopTaskSwitchMetricRecorder::DesktopTaskSwitchMetricRecorder()
+    : last_active_task_window_(nullptr) {
+  Shell::GetInstance()->activation_client()->AddObserver(this);
+}
+
+DesktopTaskSwitchMetricRecorder::~DesktopTaskSwitchMetricRecorder() {
+  Shell::GetInstance()->activation_client()->RemoveObserver(this);
+}
+
+void DesktopTaskSwitchMetricRecorder::OnWindowActivated(
+    aura::client::ActivationChangeObserver::ActivationReason reason,
+    aura::Window* gained_active,
+    aura::Window* lost_active) {
+  if (gained_active && wm::IsWindowUserPositionable(gained_active)) {
+    if (last_active_task_window_ != gained_active &&
+        reason == aura::client::ActivationChangeObserver::ActivationReason::
+                      INPUT_EVENT) {
+      Shell::GetInstance()->metrics()->RecordUserMetricsAction(
+          UMA_DESKTOP_SWITCH_TASK);
+    }
+    last_active_task_window_ = gained_active;
+  }
+}
+
+}  // namespace ash