Introduce a Gin class instead of using global functions to control gin

mojo/public/bindings/js/test/run_all_unittests.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/basictypes.h"
 #include "base/bind.h"
 #include "base/compiler_specific.h"
 #include "base/test/launcher/unit_test_launcher.h"
 #include "base/test/test_suite.h"
-#include "gin/initialize.h"
 #include "mojo/system/core_impl.h"
 
 int main(int argc, char** argv) {
   base::TestSuite test_suite(argc, argv);
 
-  gin::Initialize();
   mojo::system::CoreImpl::Init();
 
   return base::LaunchUnitTests(
       argc, argv, base::Bind(&base::TestSuite::Run,
                              base::Unretained(&test_suite)));
 }