Add desktop engagement metrics.

chrome/browser/android/metrics/uma_session_stats.cc

Index: chrome/browser/android/metrics/uma_session_stats.cc
diff --git a/chrome/browser/android/metrics/uma_session_stats.cc b/chrome/browser/android/metrics/uma_session_stats.cc
index 1f5641ff6aa4024ea17a18542b4837496377cb72..f70ee7aa68044fd5f74dd178145118948e563500 100644
--- a/chrome/browser/android/metrics/uma_session_stats.cc
+++ b/chrome/browser/android/metrics/uma_session_stats.cc
@@ -62,6 +62,9 @@ void UmaSessionStats::UmaEndSession(JNIEnv* env,
 
   if (active_session_count_ == 0) {
     base::TimeDelta duration = base::TimeTicks::Now() - session_start_time_;
+
+    // Note: This metric is recorded separately on desktop in
+    // DesktopEngagementService::EndSession.
     UMA_HISTOGRAM_LONG_TIMES("Session.TotalDuration", duration);
 
     DCHECK(g_browser_process);