Add a SafeSearch preference, policy and implementation.

chrome/browser/io_thread.cc

Index: chrome/browser/io_thread.cc
diff --git a/chrome/browser/io_thread.cc b/chrome/browser/io_thread.cc
index b8b3c9000033954a6e31e17ec8dee9cba1930b1b..726e45748035f56d5916374cf6eac7a40abcb990 100644
--- a/chrome/browser/io_thread.cc
+++ b/chrome/browser/io_thread.cc
@@ -367,6 +367,7 @@ IOThread::IOThread(
   ChromeNetworkDelegate::InitializePrefsOnUIThread(
       &system_enable_referrers_,
       NULL,
+      NULL,
       local_state);
   ssl_config_service_manager_.reset(
       SSLConfigServiceManager::CreateDefaultManager(local_state, NULL));
@@ -442,6 +443,7 @@ void IOThread::Init() {
       NULL,
       &system_enable_referrers_,
       NULL,
+      NULL,
       NULL);
   if (command_line.HasSwitch(switches::kDisableExtensionsHttpThrottling))
     network_delegate->NeverThrottleRequests();