[DesktopSessionDurationTracker] Remove visibility-switch timeout from session length.

chrome/browser/metrics/desktop_session_duration/desktop_session_duration_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_SESSION_DURATION_DESKTOP_SESSION_DURATION_TRACKER_H_
 #define CHROME_BROWSER_METRICS_DESKTOP_SESSION_DURATION_DESKTOP_SESSION_DURATION_TRACKER_H_
 
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 #include "base/time/time.h"
@@ skipping 14 matching lines @@
   // Returns true if the |DesktopSessionDurationTracker| instance has been
   // created.
   static bool IsInitialized();
 
   // Returns the |DesktopSessionDurationTracker| instance.
   static DesktopSessionDurationTracker* Get();
 
   // Called when user interaction with the browser is caught.
   void OnUserEvent();
 
-  // Called when visibility of the browser changes.
-  void OnVisibilityChanged(bool visible);
+  // Called when visibility of the browser changes. These events can be delayed
+  // due to timeout logic, the extent of which can be communicated via
+  // |time_ago|. This time is used to correct the session duration.
+  void OnVisibilityChanged(bool visible, base::TimeDelta time_ago);
 
   bool is_visible() const { return is_visible_; }
   bool in_session() const { return in_session_; }
   bool is_audio_playing() const { return is_audio_playing_; }
 
   void SetInactivityTimeoutForTesting(int seconds) {
     inactivity_timeout_ = base::TimeDelta::FromSeconds(seconds);
   }
 
  protected:
   DesktopSessionDurationTracker();
   ~DesktopSessionDurationTracker() override;
 
   // AudibleContentsTracker::Observer
   void OnAudioStart() override;
   void OnAudioEnd() override;
 
   // Decides whether session should be ended. Called when timer for inactivity
   // timeout was fired. Overridden by tests.
   virtual void OnTimerFired();
 
  private:
   // Starts timer based on |inactivity_timeout_|.
   void StartTimer(base::TimeDelta duration);
 
   // Marks the start of the session.
   void StartSession();
 
   // Ends the session and saves session information into histograms.
-  void EndSession();
+  // |time_to_discount| contains the amount of time that should be removed from
+  // the apparent session length due to timeout logic.
+  void EndSession(base::TimeDelta time_to_discount);
 
   // Sets |inactivity_timeout_| based on variation params.
   void InitInactivityTimeout();
 
   // Used for marking start if the session.
   base::TimeTicks session_start_;
 
   // Used for marking last user interaction.
   base::TimeTicks last_user_event_;
 
@@ skipping 12 matching lines @@
   AudibleContentsTracker audio_tracker_;
 
   base::WeakPtrFactory<DesktopSessionDurationTracker> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(DesktopSessionDurationTracker);
 };
 
 }  // namespace metrics
 
 #endif  // CHROME_BROWSER_METRICS_DESKTOP_SESSION_DURATION_DESKTOP_SESSION_DURATION_TRACKER_H_