bindings: Introduces ScriptWrappableBase to provide the internal pointer.

Source/bindings/core/v8/ScriptWrappable.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 "bindings/core/v8/ScriptWrappable.h"
+
+namespace blink {
+
+struct SameSizeAsScriptWrappableBase { };
+
+COMPILE_ASSERT(sizeof(ScriptWrappableBase) <= sizeof(SameSizeAsScriptWrappableBase), ScriptWrappableBase_should_stay_small);
+
+struct SameSizeAsScriptWrappable : public ScriptWrappableBase {
+    virtual ~SameSizeAsScriptWrappable() { }
+    uintptr_t m_wrapperOrTypeInfo;
+};
+
+COMPILE_ASSERT(sizeof(ScriptWrappable) <= sizeof(SameSizeAsScriptWrappable), ScriptWrappable_should_stay_small);
+
+} // namespace blink