Adding synthetic field trial for DataReductionProxyEnabled

chrome/browser/metrics/metrics_service_accessor.h

 // Copyright 2014 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.
 
 #ifndef CHROME_BROWSER_METRICS_METRICS_SERVICE_ACCESSOR_H_
 #define CHROME_BROWSER_METRICS_METRICS_SERVICE_ACCESSOR_H_
 
+#include <string>
+
 #include "base/macros.h"
 
+class MetricsService;
 class MetricsServiceObserver;
 
 // This class limits and documents access to metrics service helper methods.
 // These methods are protected so each user has to inherit own program-specific
 // specialization and enable access there by declaring friends.
 class MetricsServiceAccessor {
  protected:
   // Constructor declared as protected to enable inheritance. Decendants should
   // disallow instantiation.
   MetricsServiceAccessor() {}
 
   // Registers/unregisters |observer| to receive MetricsLog notifications
   // from metrics service.
   static void AddMetricsServiceObserver(MetricsServiceObserver* observer);
   static void RemoveMetricsServiceObserver(MetricsServiceObserver* observer);
 
+  // Registers a field trial name and group to be used to annotate a UMA report
+  // with a particular Chrome configuration state. A UMA report will be
+  // annotated with this trial group if and only if all events in the report
+  // were created after the trial is registered. Only one group name may be
+  // registered at a time for a given trial name. Only the last group name that
+  // is registered for a given trial name will be recorded. The values passed
+  // in must not correspond to any real field trial in the code.
+  static bool RegisterSyntheticFieldTrial(MetricsService* metrics_service,
+                                          const std::string& trial,
+                                          const std::string& group);
+
  private:
   DISALLOW_COPY_AND_ASSIGN(MetricsServiceAccessor);
 };
 
 #endif  // CHROME_BROWSER_METRICS_METRICS_SERVICE_ACCESSOR_H_