Add a histogram to measure number of synthetic UMA trials.

Also refactors some common code into a method in MetricsService.

BUG=400357

Review URL: https://codereview.chromium.org/612883003

Cr-Commit-Position: refs/heads/master@{#297302}

components/metrics/metrics_service.cc

@@ -766,10 +766,7 @@ void MetricsService::CloseCurrentLog() {
   // MetricsLog class.
   MetricsLog* current_log = log_manager_.current_log();
   DCHECK(current_log);
-  std::vector<variations::ActiveGroupId> synthetic_trials;
-  GetCurrentSyntheticFieldTrials(&synthetic_trials);
-  current_log->RecordEnvironment(
-      metrics_providers_.get(), synthetic_trials, GetInstallDate());
+  RecordCurrentEnvironment(current_log);
   base::TimeDelta incremental_uptime;
   base::TimeDelta uptime;
   GetUptimes(local_state_, &incremental_uptime, &uptime);
@@ -972,11 +969,7 @@ void MetricsService::PrepareInitialStabilityLog() {
 void MetricsService::PrepareInitialMetricsLog() {
   DCHECK(state_ == INIT_TASK_DONE || state_ == SENDING_INITIAL_STABILITY_LOG);
 
-  std::vector<variations::ActiveGroupId> synthetic_trials;
-  GetCurrentSyntheticFieldTrials(&synthetic_trials);
-  initial_metrics_log_->RecordEnvironment(metrics_providers_.get(),
-                                          synthetic_trials,
-                                          GetInstallDate());
+  RecordCurrentEnvironment(initial_metrics_log_.get());
   base::TimeDelta incremental_uptime;
   base::TimeDelta uptime;
   GetUptimes(local_state_, &incremental_uptime, &uptime);
@@ -1177,6 +1170,15 @@ scoped_ptr<MetricsLog> MetricsService::CreateLog(MetricsLog::LogType log_type) {
                                         local_state_));
 }
 
+void MetricsService::RecordCurrentEnvironment(MetricsLog* log) {
+  std::vector<variations::ActiveGroupId> synthetic_trials;
+  GetCurrentSyntheticFieldTrials(&synthetic_trials);
+  log->RecordEnvironment(metrics_providers_.get(), synthetic_trials,
+                         GetInstallDate());
+  UMA_HISTOGRAM_COUNTS_100("UMA.SyntheticTrials.Count",
+                           synthetic_trials.size());
+}
+
 void MetricsService::RecordCurrentHistograms() {
   DCHECK(log_manager_.current_log());
   histogram_snapshot_manager_.PrepareDeltas(

components/metrics/metrics_service.h

@@ -364,6 +364,9 @@ class MetricsService : public base::HistogramFlattener {
   // Creates a new MetricsLog instance with the given |log_type|.
   scoped_ptr<MetricsLog> CreateLog(MetricsLog::LogType log_type);
 
+  // Records the current environment (system profile) in |log|.
+  void RecordCurrentEnvironment(MetricsLog* log);
+
   // Record complete list of histograms into the current log.
   // Called when we close a log.
   void RecordCurrentHistograms();

tools/metrics/histograms/histograms.xml

@@ -35618,6 +35618,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="UMA.SyntheticTrials.Count">
+  <owner>asvitkine@chromium.org</owner>
+  <summary>
+    The number of synthetic field trials added to the UMA log when the system
+    profile is recorded. Since this is done prior to capturing the histograms
+    from the current process, this will generally be logged once per UMA log.
+  </summary>
+</histogram>
+
 <histogram name="UMA.Unacceptable_Log_Discarded">
   <obsolete>
     Deprecated as of May, 2012 (i.e. Chrome 21+).  Replaced by the