Add desktop engagement metrics.

chrome/browser/android/metrics/uma_session_stats.cc

 // Copyright 2015 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.
 
 #include "chrome/browser/android/metrics/uma_session_stats.h"
 
 #include "base/android/jni_array.h"
 #include "base/android/jni_string.h"
 #include "base/command_line.h"
 #include "base/metrics/histogram.h"
@@ skipping 44 matching lines @@
   ++active_session_count_;
 }
 
 void UmaSessionStats::UmaEndSession(JNIEnv* env,
                                     const JavaParamRef<jobject>& obj) {
   --active_session_count_;
   DCHECK_GE(active_session_count_, 0);
 
   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);
     // Tell the metrics service it was cleanly shutdown.
     metrics::MetricsService* metrics = g_browser_process->metrics_service();
     if (metrics) {
       metrics->OnAppEnterBackground();
     }
   }
 }
@@ skipping 129 matching lines @@
   // We should have only one UmaSessionStats instance.
   DCHECK(!g_uma_session_stats);
   g_uma_session_stats = new UmaSessionStats();
   return reinterpret_cast<intptr_t>(g_uma_session_stats);
 }
 
 // Register native methods
 bool RegisterUmaSessionStats(JNIEnv* env) {
   return RegisterNativesImpl(env);
 }