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 {
+
+namespace {
+ChromeVisibilityObserver* g_instance = nullptr;
+}
+
+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() {

[Alexei Svitkine (slow), 2016/07/28 21:31:45]

Sorry, I missed this before, but can you change this and audible tracker be
members of DesktopEngagementService? Then, they don't need to be globals or have
Initialize() functions and can just be members of that class. (Don't even need
to have them be allocated with new, can just be direct members.)

[gayane -on leave until 09-2017, 2016/07/29 19:37:39]

Done.

+  g_instance = new ChromeVisibilityObserver();
+}
+
+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 there are no browser instances left then we should notify that browser
+  // is not visible anymore immediately without waiting.
+  if (BrowserList::GetInstance()->empty()) {
+    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);
+}
+
+}  // namespace metrics