Create a synthetic field trial for precache.

chrome/browser/precache/precache_manager_factory.cc

Index: chrome/browser/precache/precache_manager_factory.cc
diff --git a/chrome/browser/precache/precache_manager_factory.cc b/chrome/browser/precache/precache_manager_factory.cc
index c4d533fbb1ac49e1321d5a73f5834b327be1f0e6..feec9c9fc5c0a28833f805b9913f4a65582d6e3e 100644
--- a/chrome/browser/precache/precache_manager_factory.cc
+++ b/chrome/browser/precache/precache_manager_factory.cc
@@ -11,6 +11,7 @@
 #include "chrome/browser/history/history_service_factory.h"
 #include "chrome/browser/net/spdyproxy/data_reduction_proxy_chrome_settings.h"
 #include "chrome/browser/net/spdyproxy/data_reduction_proxy_chrome_settings_factory.h"
+#include "chrome/browser/precache/precache_util.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/sync/profile_sync_service_factory.h"
 #include "components/keyed_service/content/browser_context_dependency_manager.h"
@@ -60,7 +61,8 @@ KeyedService* PrecacheManagerFactory::BuildServiceInstanceFor(
           ServiceAccessType::IMPLICIT_ACCESS),
       DataReductionProxyChromeSettingsFactory::GetForBrowserContext(
           browser_context),
-      db_path, std::move(precache_database));
+      db_path, std::move(precache_database),
+      base::Bind(&precache::RegisterPrecacheSyntheticFieldTrial));
 }
 
 }  // namespace precache