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 dbe8e404ecdde54952c8346ce6556d2b834a247f..86947b8894ebde357e532d4bee67c2104a7609ee 100644
--- a/chrome/browser/precache/precache_manager_factory.cc
+++ b/chrome/browser/precache/precache_manager_factory.cc
@@ -8,6 +8,7 @@
 
 #include "base/files/file_path.h"
 #include "chrome/browser/history/history_service_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"
@@ -52,7 +53,8 @@ KeyedService* PrecacheManagerFactory::BuildServiceInstanceFor(
       HistoryServiceFactory::GetForProfile(
           Profile::FromBrowserContext(browser_context),
           ServiceAccessType::IMPLICIT_ACCESS),
-      db_path, std::move(precache_database));
+      db_path, std::move(precache_database),
+      base::Bind(&precache::RegisterPrecacheSyntheticFieldTrial));
 }
 
 }  // namespace precache