[bindings] Migrate V8TestingScope out of V8Binding.h/cpp

Source/core/testing/PrivateScriptTestTest.cpp

 // Copyright 2014 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 "config.h"
 #include "core/testing/PrivateScriptTest.h"
 
 #include "bindings/core/v8/PrivateScriptRunner.h"
 #include "bindings/core/v8/V8Binding.h"
 #include "bindings/core/v8/V8PrivateScriptTest.h"
+#include "bindings/core/v8/V8TestingScope.h"
 #include "core/testing/DummyPageHolder.h"
 
 #include <gtest/gtest.h>
 
 // PrivateScriptTest.js is available only in debug builds.
 #ifndef NDEBUG
 namespace blink {
 
 namespace {
 
@@ skipping 41 matching lines @@
     EXPECT_TRUE(success);
     success = V8PrivateScriptTest::PrivateScript::stringAttributeForPrivateScriptOnlyAttributeGetter(frame(), privateScriptTest(), &result);
     EXPECT_TRUE(success);
     EXPECT_EQ(result, "foo");
 }
 
 } // namespace
 
 } // namespace blink
 #endif