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

mojo/apps/js/main.cc

Index: mojo/apps/js/main.cc
diff --git a/mojo/apps/js/main.cc b/mojo/apps/js/main.cc
index 6f8cdb367ada153fd44ba02eb4abb577174ca088..1d5e896ca9248e567845c76ae05d144446210827 100644
--- a/mojo/apps/js/main.cc
+++ b/mojo/apps/js/main.cc
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "gin/initialize.h"
+#include "gin/gin.h"
 #include "mojo/public/system/core.h"
 #include "mojo/public/system/macros.h"
 
@@ -18,7 +18,7 @@
 
 extern "C" MOJO_APPS_JS_EXPORT MojoResult CDECL MojoMain(
     mojo::Handle pipe) {
-  gin::Initialize();
+  gin::Gin instance;
   // TODO(abarth): Load JS off the network and execute it.
   return MOJO_RESULT_OK;
 }