cros: Add enterprise user session metrics

chrome/browser/chromeos/session_length_limiter.h

Index: chrome/browser/chromeos/session_length_limiter.h
diff --git a/chrome/browser/chromeos/session_length_limiter.h b/chrome/browser/chromeos/session_length_limiter.h
index a52a2c3a7c88466e58517fb866f13f66a756b74b..2e6e52300a0b8529a03d5cabd1fa6c5d898c3ce9 100644
--- a/chrome/browser/chromeos/session_length_limiter.h
+++ b/chrome/browser/chromeos/session_length_limiter.h
@@ -37,6 +37,10 @@ class SessionLengthLimiter : public ui::UserActivityObserver {
   SessionLengthLimiter(Delegate* delegate, bool browser_restarted);
   ~SessionLengthLimiter() override;
 
+  // Returns the duration between |session_start_time_| and now if there is a
+  // valid |session_start_time_|. Otherwise, returns 0.
+  base::TimeDelta GetSessionDuration() const;
+
   // ui::UserActivityObserver:
   void OnUserActivity(const ui::Event* event) override;