Add desktop engagement metrics.

chrome/browser/metrics/desktop_engagement/audible_contents_tracker.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/audible_contents_tracker.h"
+
+#include "chrome/browser/ui/browser.h"
+#include "chrome/browser/ui/browser_list.h"
+#include "chrome/browser/ui/tabs/tab_strip_model.h"
+
+namespace metrics {
+
+AudibleContentsTracker::AudibleContentsTracker(Observer* observer)
+    : observer_(observer) {
+  auto browser_list = BrowserList::GetInstance();
+  for (const auto browser : *browser_list)

[Alexei Svitkine (slow), 2016/07/26 19:20:36]

Nit: See recent chromium-dev discussion, basically the guidance is to explicitly
put a * on pointer types when using auto. However, in this case it should be
short enough to just spell out the type, so please do that.

[gayane -on leave until 09-2017, 2016/07/26 21:41:18]

Done.

+    browser->tab_strip_model()->AddObserver(this);
+  browser_list->AddObserver(this);
+}
+
+AudibleContentsTracker::~AudibleContentsTracker() {}
+
+void AudibleContentsTracker::OnBrowserAdded(Browser* browser) {
+  browser->tab_strip_model()->AddObserver(this);
+}
+
+void AudibleContentsTracker::OnBrowserRemoved(Browser* browser) {
+  browser->tab_strip_model()->RemoveObserver(this);
+}
+
+void AudibleContentsTracker::TabClosingAt(TabStripModel* model,
+                                          content::WebContents* web_contents,
+                                          int index) {
+  RemoveAudibleWebContents(web_contents);
+}
+
+void AudibleContentsTracker::TabChangedAt(content::WebContents* web_contents,
+                                          int index,
+                                          TabChangeType change_type) {
+  // Ignore 'loading' and 'title' changes.
+  if (change_type != TabStripModelObserver::ALL)
+    return;
+
+  if (web_contents->WasRecentlyAudible()) {

[Alexei Svitkine (slow), 2016/07/26 19:20:36]

Nit: No {}'s

[gayane -on leave until 09-2017, 2016/07/26 21:41:18]

Done.

+    AddAudibleWebContents(web_contents);
+  } else {
+    RemoveAudibleWebContents(web_contents);
+  }
+}
+
+void AudibleContentsTracker::TabReplacedAt(
+    TabStripModel* model,
+    content::WebContents* old_web_contents,
+    content::WebContents* new_web_contents,
+    int index) {
+  RemoveAudibleWebContents(old_web_contents);
+  if (new_web_contents->WasRecentlyAudible())
+    AddAudibleWebContents(new_web_contents);
+}
+
+void AudibleContentsTracker::AddAudibleWebContents(
+    content::WebContents* web_contents) {
+  // The first web contents to become audible indicates that audio has started.
+  bool added = audible_contents_.insert(web_contents).second;
+  if (added && audible_contents_.size() == 1)
+    observer_->OnAudioStart();
+}
+
+void AudibleContentsTracker::RemoveAudibleWebContents(
+    content::WebContents* web_contents) {
+  bool removed = (audible_contents_.erase(web_contents) == 1);

[Alexei Svitkine (slow), 2016/07/26 19:20:36]

Add a comment about this logic please.

[gayane -on leave until 09-2017, 2016/07/26 21:41:18]

Done.

+  if (removed && audible_contents_.empty())
+    observer_->OnAudioEnd();
+}
+
+}  // namespace metrics