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

ash/metrics/screen_event_task_switch_metric_recorder.cc

Index: ash/metrics/screen_event_task_switch_metric_recorder.cc
diff --git a/ash/metrics/screen_event_task_switch_metric_recorder.cc b/ash/metrics/screen_event_task_switch_metric_recorder.cc
new file mode 100644
index 0000000000000000000000000000000000000000..ef365d99127cff0ecdcf90479e2e049a5f256aa6
--- /dev/null
+++ b/ash/metrics/screen_event_task_switch_metric_recorder.cc
@@ -0,0 +1,56 @@
+// 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/screen_event_task_switch_metric_recorder.h"
+
+#include "ash/metrics/user_metrics_recorder.h"
+#include "ash/shell.h"
+#include "ash/wm/mru_window_tracker.h"
+#include "ui/wm/public/activation_client.h"
+
+namespace ash {
+
+namespace {
+
+// Returns true if |window| is considered a task window.
+bool IsATaskWindow(aura::Window* window) {
+  return Shell::GetInstance()
+      ->metrics()
+      ->task_switch_metrics_recorder()
+      .IsATaskWindow(window);
+}
+
+// Returns the last active task window if one exists or nullptr otherwise.
+aura::Window* GetLastActiveTaskWindow() {
+  MruWindowTracker::WindowList window_list =
+      Shell::GetInstance()->mru_window_tracker()->BuildMruWindowList();
+  return window_list.empty() ? nullptr : window_list[0];
+}
+
+}  // namespace
+
+ScreenEventTaskSwitchMetricRecorder::ScreenEventTaskSwitchMetricRecorder(
+    UserMetricsRecorder* user_metrics_recorder)
+    : user_metrics_recorder_(user_metrics_recorder) {
+}
+
+ScreenEventTaskSwitchMetricRecorder::~ScreenEventTaskSwitchMetricRecorder() {
+}
+
+void ScreenEventTaskSwitchMetricRecorder::
+    BeforeWindowActivatedByPreTargetHandler() {
+  active_window_before_activation_ = GetLastActiveTaskWindow();
+}
+
+void ScreenEventTaskSwitchMetricRecorder::OnWindowActivatedByPreTargetHandler(
+    aura::Window* gained_active,
+    aura::Window* lost_active) {
+  if (IsATaskWindow(gained_active) &&
+      active_window_before_activation_ != gained_active) {
+    Shell::GetInstance()->metrics()->RecordUserMetricsAction(
+        UMA_SCREEN_EVENT_SWITCH_TASK);
+  }
+}
+
+}  // namespace ash