Add Desktop Engagement Service

chrome/browser/metrics/desktop_engagement_service.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_service.h"
+
+#include "base/bind.h"
+#include "base/metrics/histogram_macros.h"
+
+namespace {
+
+DesktopEngagementService* g_instance = nullptr;
+
+}  // namespace
+
+// static
+void DesktopEngagementService::Initialize() {
+  g_instance = new DesktopEngagementService;
+}
+
+void DesktopEngagementService::StartTimer(base::TimeDelta duration) {
+  timer_.Start(FROM_HERE, duration,
+               base::Bind(&DesktopEngagementService::OnTimerFired,
+                          base::Unretained(this)));

[Alexei Svitkine (slow), 2016/06/29 18:27:22]

Use a WeakPointerFactory member and pass a weak pointer instead of
base::Unretained()

+}
+
+void DesktopEngagementService::OnUserEvent() {
+  last_user_event_ = base::TimeTicks::Now();
+  // This may start session.
+  if (!in_session)
+    StartSession();
+}
+
+void DesktopEngagementService::OnAudioStart() {
+  // This may start session.
+  is_audio_playing_ = true;
+  if (!in_session)
+    StartSession();
+}
+
+void DesktopEngagementService::OnAudioEnd() {
+  is_audio_playing_ = false;
+
+  // If the timer is not running, this means that no user events happened in the
+  // last 5 minutes so the session can be terminated.
+  if (!timer_.IsRunning())
+    EndSession();
+}
+
+DesktopEngagementService::DesktopEngagementService()
+    : session_start_(base::TimeTicks::Now()), last_user_event_(session_start_) {
+  StartSession();
+  // TODO: Add user events observers here, or add a static Get() method to
+  //       DesktopEngagementService.
+}
+
+void DesktopEngagementService::OnTimerFired() {
+  base::TimeDelta remaining_timer = base::TimeTicks::Now() - last_user_event_;
+  if (remaining_timer < base::TimeDelta::FromMinutes(5)) {
+    StartTimer(remaining_timer);
+    return;
+  }
+
+  // No user events happened in the last 5 min. Terminate the session now.
+  if (!is_audio_playing_)
+    EndSession();
+}
+
+void DesktopEngagementService::StartSession() {
+  in_session = true;
+  session_start_ = base::TimeTicks::Now();
+  StartTimer(base::TimeDelta::FromMinutes(5));
+}
+
+void DesktopEngagementService::EndSession() {
+  in_session = false;
+  base::TimeDelta delta = base::TimeTicks::Now() - session_start_;
+
+  constexpr unsigned kNumTimeSlices = 60 / 5 * 24;
+  UMA_HISTOGRAM_CUSTOM_TIMES("Session.TotalDuration.Desktop", delta,

[Alexei Svitkine (slow), 2016/06/29 18:27:22]

Add to histograms.xml please. :)

Or just use Session.TotalDuration for the name, which I think we have for mobile
already?

+                             base::TimeDelta::FromMilliseconds(1),
+                             base::TimeDelta::FromHours(24), kNumTimeSlices);
+}