Rename DesktopEngagement* to DesktopSessionDuration*.

chrome/browser/metrics/desktop_session_duration/chrome_visibility_observer.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_CHROME_VISIBILITY_OBSERVER_H_
-#define CHROME_BROWSER_METRICS_DESKTOP_ENGAGEMENT_CHROME_VISIBILITY_OBSERVER_H_
+#ifndef CHROME_BROWSER_METRICS_DESKTOP_SESSION_DURATION_CHROME_VISIBILITY_OBSERVER_H_
+#define CHROME_BROWSER_METRICS_DESKTOP_SESSION_DURATION_CHROME_VISIBILITY_OBSERVER_H_
 
 #include "base/memory/weak_ptr.h"
 #include "chrome/browser/ui/browser_list_observer.h"
 
 namespace metrics {
-
 // Observer for tracking browser visibility events.
 class ChromeVisibilityObserver : public chrome::BrowserListObserver {
  public:
   ChromeVisibilityObserver();
   ~ChromeVisibilityObserver() override;
 
  protected:
-  // Notifies |DesktopEngagementService| of visibility changes. Overridden by
-  // tests.
+  // Notifies |DesktopSessionDurationTracker| of visibility changes. Overridden
+  // by tests.
   virtual void SendVisibilityChangeEvent(bool active);
 
  private:
   // Cancels visibility change in case when the browser becomes visible after a
   // short gap.
   void CancelVisibilityChange();
 
   // chrome::BrowserListObserver:
   void OnBrowserSetLastActive(Browser* browser) override;
   void OnBrowserNoLongerActive(Browser* browser) override;
   void OnBrowserRemoved(Browser* browser) override;
 
   // Sets |visibility_gap_timeout_| based on variation params.
   void InitVisibilityGapTimeout();
 
   // Timeout interval for waiting after loss of visibility. This allows merging
   // two visibility session if they happened very shortly after each other, for
   // example, when user switching between two browser windows.
   base::TimeDelta visibility_gap_timeout_;
 
   base::WeakPtrFactory<ChromeVisibilityObserver> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ChromeVisibilityObserver);
 };
 
 }  // namespace metrics
 
-#endif  // CHROME_BROWSER_METRICS_DESKTOP_ENGAGEMENT_CHROME_VISIBILITY_OBSERVER_H_
+#endif  // CHROME_BROWSER_METRICS_DESKTOP_SESSION_DURATION_CHROME_VISIBILITY_OBSERVER_H_