Rename DesktopEngagement* to DesktopSessionDuration*.

chrome/browser/metrics/desktop_session_duration/audible_contents_tracker.h

 // 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.
 
-#ifndef CHROME_BROWSER_METRICS_DESKTOP_ENGAGEMENT_AUDIBLE_CONTENTS_TRACKER_H_
-#define CHROME_BROWSER_METRICS_DESKTOP_ENGAGEMENT_AUDIBLE_CONTENTS_TRACKER_H_
+#ifndef CHROME_BROWSER_METRICS_DESKTOP_SESSION_DURATION_AUDIBLE_CONTENTS_TRACKER_H_
+#define CHROME_BROWSER_METRICS_DESKTOP_SESSION_DURATION_AUDIBLE_CONTENTS_TRACKER_H_
 
 #include <set>
 
 #include "base/callback.h"
 #include "chrome/browser/ui/browser_list_observer.h"
 #include "chrome/browser/ui/tabs/tab_strip_model_observer.h"
 
 namespace metrics {
 
 // BrowserList / TabStripModelObserver used for tracking audio status.
 class AudibleContentsTracker : public chrome::BrowserListObserver,
                                public TabStripModelObserver {
  public:
   // Interface for an observer of the AudibleContentsTracker. The only client
-  // of this class is the DesktopEngagementService, but an observer interface
-  // has been created for ease of testing.
+  // of this class is the DesktopSessionDurationTracker, but an observer
+  // interface has been created for ease of testing.
   class Observer {

[sky, 2016/09/15 03:04:05]

for a later patch: Style guide says observers should be in their own header.

    public:
     Observer() {}
     virtual ~Observer() {}
 
     // Invoked when a first audio source starts playing after a period of no
     // audio sources.
     virtual void OnAudioStart() = 0;
 
     // Invoked when all audio sources stop playing.
     virtual void OnAudioEnd() = 0;
@@ skipping 32 matching lines @@
   Observer* observer_;
 
   // The set of WebContents that are currently playing audio.
   std::set<content::WebContents*> audible_contents_;
 
   DISALLOW_COPY_AND_ASSIGN(AudibleContentsTracker);
 };
 
 }  // namespace metrics
 
-#endif  // CHROME_BROWSER_METRICS_DESKTOP_ENGAGEMENT_AUDIBLE_CONTENTS_TRACKER_H_
+#endif  // CHROME_BROWSER_METRICS_DESKTOP_SESSION_DURATION_AUDIBLE_CONTENTS_TRACKER_H_