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 ddc6ac4fa390b440e6a777600c6803ded2a6306d..3fe2765bc6a2f5fa387fd30b48e2515b3531d1fc 100644
--- a/chrome/browser/android/metrics/uma_session_stats.cc
+++ b/chrome/browser/android/metrics/uma_session_stats.cc
@@ -61,6 +61,9 @@ void UmaSessionStats::UmaEndSession(JNIEnv* env,
 
   if (active_session_count_ == 0) {
     base::TimeDelta duration = base::TimeTicks::Now() - session_start_time_;
+
+    // This metric is recorded separately for desktops in

[Alexei Svitkine (slow), 2016/07/28 18:37:22]

Nit: "for desktops" -> "on desktop"

Add a "Note: " at the front of the comment.

[gayane -on leave until 09-2017, 2016/07/28 21:21:34]

Done.

+    // DesktopEngagementService::EndSession.
     UMA_HISTOGRAM_LONG_TIMES("Session.TotalDuration", duration);
 
     DCHECK(g_browser_process);