Rename DesktopEngagement* to DesktopSessionDuration*.

chrome/browser/metrics/desktop_engagement/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_
-
-#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.
-  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_