Create a synthetic field trial for precache.

chrome/browser/metrics/chrome_metrics_service_accessor.h

Index: chrome/browser/metrics/chrome_metrics_service_accessor.h
diff --git a/chrome/browser/metrics/chrome_metrics_service_accessor.h b/chrome/browser/metrics/chrome_metrics_service_accessor.h
index 04df2f9a93a6cd4b4125a4f77b0865b34e5b0d98..02a2fdac75e42ab050f4dee2ce13f7633bbd213b 100644
--- a/chrome/browser/metrics/chrome_metrics_service_accessor.h
+++ b/chrome/browser/metrics/chrome_metrics_service_accessor.h
@@ -49,6 +49,10 @@ namespace options {
 class BrowserOptionsHandler;
 }
 
+namespace precache {
+void RegisterPrecacheSyntheticFieldTrial(base::Time);
+}
+
 namespace prerender {
 bool IsOmniboxEnabled(Profile* profile);
 }
@@ -107,6 +111,7 @@ class ChromeMetricsServiceAccessor : public metrics::MetricsServiceAccessor {
       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;