logging: restore call to log_message_handler::Install()

I missed this when moving shared components to SystemRewriteDriverFactory;
this call used to be in NgxRewriteDriverFactory::RootInit.  Without it we only
log VLOG(0), not VLOG(1) and VLOG(2) the way Apache does.

Fixes #518.

src/ngx_rewrite_driver_factory.cc

@@ -206,6 +206,7 @@ void NgxRewriteDriverFactory::StartThreads() {
 }
 
 void NgxRewriteDriverFactory::LoggingInit(ngx_log_t* log) {
+  net_instaweb::log_message_handler::Install(log);
   if (install_crash_handler_) {
     NgxMessageHandler::InstallCrashHandler(log);
   }