Create a synthetic field trial for precache.

chrome/browser/metrics/chrome_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_CHROME_METRICS_SERVICE_ACCESSOR_H_
 #define CHROME_BROWSER_METRICS_CHROME_METRICS_SERVICE_ACCESSOR_H_
 
 #include <stdint.h>
 #include <string>
 #include <vector>
@@ skipping 31 matching lines @@
 namespace extensions {
 class ChromeExtensionWebContentsObserver;
 class ChromeMetricsPrivateDelegate;
 class FileManagerPrivateIsUMAEnabledFunction;
 }
 
 namespace options {
 class BrowserOptionsHandler;
 }
 
+namespace precache {
+void RegisterPrecacheSyntheticFieldTrial(base::Time);
+}
+
 namespace prerender {
 bool IsOmniboxEnabled(Profile* profile);
 }
 
 namespace safe_browsing {
 class DownloadSBClient;
 class IncidentReportingService;
 class ReporterRunner;
 class SafeBrowsingService;
 class SafeBrowsingUIManager;
@@ skipping 38 matching lines @@
   friend class ChromeRenderMessageFilter;
   friend class DataReductionProxyChromeSettings;
   friend class domain_reliability::DomainReliabilityServiceFactory;
   friend class extensions::ChromeExtensionWebContentsObserver;
   friend class extensions::ChromeMetricsPrivateDelegate;
   friend class extensions::FileManagerPrivateIsUMAEnabledFunction;
   friend void ChangeMetricsReportingStateWithReply(
       bool,
       const OnMetricsReportingCallbackType&);
   friend class options::BrowserOptionsHandler;
+  friend void precache::RegisterPrecacheSyntheticFieldTrial(base::Time);
   friend bool prerender::IsOmniboxEnabled(Profile* profile);
   friend class settings::MetricsReportingHandler;
   friend class speech::ChromeSpeechRecognitionManagerDelegate;
   friend class system_logs::ChromeInternalLogSource;
   friend class UmaSessionStats;
   friend class safe_browsing::DownloadSBClient;
   friend class safe_browsing::IncidentReportingService;
   friend class safe_browsing::ReporterRunner;
   friend class safe_browsing::SRTFetcher;
   friend class safe_browsing::SRTGlobalError;
@@ skipping 29 matching lines @@
   // with g_browser_process->metrics_service(). See that function's declaration
   // for details.
   static bool RegisterSyntheticFieldTrialWithNameHash(
       uint32_t trial_name_hash,
       const std::string& group_name);
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(ChromeMetricsServiceAccessor);
 };
 
 #endif  // CHROME_BROWSER_METRICS_CHROME_METRICS_SERVICE_ACCESSOR_H_