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

gin/runner_unittest.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 "gin/runner.h"
 
 #include "base/compiler_specific.h"
 #include "gin/converter.h"
+#include "gin/gin.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 using v8::Isolate;
 using v8::Object;
 using v8::Script;
 using v8::String;
 
 namespace gin {
 
 TEST(RunnerTest, Run) {
   std::string source = "this.result = 'PASS';\n";
 
+  gin::Gin instance;
+
   RunnerDelegate delegate;
-  Isolate* isolate = Isolate::GetCurrent();
+  Isolate* isolate = instance.isolate();
   Runner runner(&delegate, isolate);
   Runner::Scope scope(&runner);
   runner.Run(source);
 
   std::string result;
   EXPECT_TRUE(Converter<std::string>::FromV8(
       runner.global()->Get(StringToV8(isolate, "result")),
       &result));
   EXPECT_EQ("PASS", result);
 }
 
 }  // namespace gin