Add desktop engagement metrics.

chrome/browser/metrics/chrome_visibility_observer.cc

+// Copyright (c) 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/chrome_visibility_observer.h"
+
+#include "base/memory/singleton.h"
+#include "chrome/browser/metrics/desktop_engagement_service.h"
+#include "chrome/browser/ui/browser.h"
+#include "chrome/browser/ui/browser_list.h"
+
+namespace metrics {
+
+namespace {
+
+const int kVisibilityGapTolerance = 5;  // seconds, can make it a finch param.

[Patrick Monette, 2016/07/13 16:06:24]

constexpr base::TimeDelta kVisibilityGapTolerance =
base::TimeDelta::FromSeconds(5);

The constant is only used in one function, maybe move it there?

[gayane -on leave until 09-2017, 2016/07/20 20:22:02]

Logic changed a bit. Please have a look/

+
+}  // namespace
+
+ChromeVisibilityObserver::ChromeVisibilityObserver()
+    : weak_factory_(this) {}
+
+ChromeVisibilityObserver::~ChromeVisibilityObserver() {
+  BrowserList::RemoveObserver(this);
+}
+
+void ChromeVisibilityObserver::SendVisibilityChangeEvent(bool active) {
+  DesktopEngagementService::Get()->OnVisibilityChanged(active);
+}
+
+void ChromeVisibilityObserver::CancelVisibilityChange() {
+  weak_factory_.InvalidateWeakPtrs();
+}
+
+// static
+ChromeVisibilityObserver* ChromeVisibilityObserver::GetInstance() {

[chrisha, 2016/07/14 13:58:48]

Does this need to be a singleton? Maybe this can simply be owned by the
DesktopEngagementService, like the AudibleContentsTracker?

[gayane -on leave until 09-2017, 2016/07/20 20:22:02]

Agreed. It made more sense before. I have removed the singlton

+  return base::Singleton<ChromeVisibilityObserver>::get();
+}
+
+// static
+void ChromeVisibilityObserver::Initialize() {
+  BrowserList::AddObserver(GetInstance());
+}
+
+void ChromeVisibilityObserver::OnBrowserSetLastActive(Browser* browser) {
+  if (weak_factory_.HasWeakPtrs())
+    CancelVisibilityChange();
+  else
+    SendVisibilityChangeEvent(true);
+}
+
+void ChromeVisibilityObserver::OnBrowserNoLongerActive(Browser* browser) {
+  base::MessageLoop::current()->PostDelayedTask(
+      FROM_HERE,
+      base::Bind(&ChromeVisibilityObserver::SendVisibilityChangeEvent,
+                 weak_factory_.GetWeakPtr(), false),
+      base::TimeDelta::FromSeconds(kVisibilityGapTolerance));
+}
+
+void ChromeVisibilityObserver::OnBrowserRemoved(Browser* browser) {
+  if (BrowserList::GetInstance()->empty()) {
+    CancelVisibilityChange();
+    SendVisibilityChangeEvent(false);
+  }
+}
+
+}  // namespace metrics