Add desktop engagement metrics.

chrome/browser/metrics/desktop_engagement/chrome_visibility_observer.cc

+// Copyright 2016 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/metrics/desktop_engagement/chrome_visibility_observer.h"
+
+#include "base/memory/singleton.h"
+#include "base/strings/string_number_conversions.h"
+#include "base/threading/thread_task_runner_handle.h"
+#include "chrome/browser/metrics/desktop_engagement/desktop_engagement_service.h"
+#include "chrome/browser/ui/browser.h"
+#include "chrome/browser/ui/browser_list.h"
+#include "components/variations/variations_associated_data.h"
+
+namespace metrics {
+
+ChromeVisibilityObserver::ChromeVisibilityObserver() : weak_factory_(this) {
+  BrowserList::AddObserver(this);
+  InitVisibilityGapTimeout();
+}
+
+ChromeVisibilityObserver::~ChromeVisibilityObserver() {
+  BrowserList::RemoveObserver(this);
+}
+
+void ChromeVisibilityObserver::SendVisibilityChangeEvent(bool active) {
+  DesktopEngagementService::Get()->OnVisibilityChanged(active);
+}
+
+void ChromeVisibilityObserver::CancelVisibilityChange() {
+  weak_factory_.InvalidateWeakPtrs();
+}
+
+// static
+void ChromeVisibilityObserver::Initialize() {
+  ChromeVisibilityObserver observer;
+}
+
+void ChromeVisibilityObserver::OnBrowserSetLastActive(Browser* browser) {
+  if (weak_factory_.HasWeakPtrs())
+    CancelVisibilityChange();
+  else
+    SendVisibilityChangeEvent(true);
+}
+
+void ChromeVisibilityObserver::OnBrowserNoLongerActive(Browser* browser) {
+  base::ThreadTaskRunnerHandle::Get()->PostDelayedTask(
+      FROM_HERE,
+      base::Bind(&ChromeVisibilityObserver::SendVisibilityChangeEvent,
+                 weak_factory_.GetWeakPtr(), false),
+      visibility_gap_timeout_);
+}
+
+void ChromeVisibilityObserver::OnBrowserRemoved(Browser* browser) {
+  if (BrowserList::GetInstance()->empty()) {

[Alexei Svitkine (slow), 2016/07/26 22:00:40]

Add a comment explaining this logic.

[gayane -on leave until 09-2017, 2016/07/27 21:10:24]

Done.

+    CancelVisibilityChange();
+    SendVisibilityChangeEvent(false);
+  }
+}
+
+void ChromeVisibilityObserver::InitVisibilityGapTimeout() {
+  const int kDefaultVisibilityGapTimeout = 3;
+
+  int timeout_seconds = kDefaultVisibilityGapTimeout;
+  std::string param_value = variations::GetVariationParamValue(
+      "DesktopEngagement", "visibility_gap_timeout");
+  if (!param_value.empty())
+    base::StringToInt(param_value, &timeout_seconds);
+
+  visibility_gap_timeout_ = base::TimeDelta::FromSeconds(timeout_seconds);
+}

[Alexei Svitkine (slow), 2016/07/26 22:00:40]

Add an empty line below this.

[gayane -on leave until 09-2017, 2016/07/27 21:10:24]

Done.

+}  // namespace metrics