bindings: Introduces ScriptWrappableBase to provide the internal pointer.

Source/bindings/core/v8/ScriptWrappable.h

Index: Source/bindings/core/v8/ScriptWrappable.h
diff --git a/Source/bindings/core/v8/ScriptWrappable.h b/Source/bindings/core/v8/ScriptWrappable.h
index b0e176c1f05a95e8a3b42cbfc1207c60a1992462..38ecd9bc24f6816dc9df1ac54e3368b84aded485 100644
--- a/Source/bindings/core/v8/ScriptWrappable.h
+++ b/Source/bindings/core/v8/ScriptWrappable.h
@@ -46,6 +46,30 @@ template <class C> inline void initializeScriptWrappableHelper(C* object)
 namespace blink {
 
 /**
+ * The base class of all wrappable objects.
+ *
+ * This class provides the internal pointer to be stored in the wrapper objects,
+ * and its conversions from / to the DOM instances.
+ *
+ * Note that this class must not have vtbl (any virtual function) or any member
+ * variable which increase the size of instances.  Some of the classes sensitive
+ * to the size inherit from this class.  So this class must be zero size.
+ */
+class ScriptWrappableBase {
+public:
+    template <class T> static T* fromInternalPointer(void* internalPointer)
+    {
+        // Check if T* is castable to ScriptWrappableBase*, which means T
+        // doesn't have two or more ScriptWrappableBase as superclasses.
+        // If T has two ScriptWrappableBase as superclasses, conversions
+        // from T* to ScriptWrappableBase* are ambiguous.
+        ASSERT(static_cast<ScriptWrappableBase*>(static_cast<T*>(static_cast<ScriptWrappableBase*>(internalPointer))));
+        return static_cast<T*>(static_cast<ScriptWrappableBase*>(internalPointer));
+    }
+    void* toInternalPointer() { return this; }
+};
+
+/**
  * ScriptWrappable wraps a V8 object and its WrapperTypeInfo.
  *
  * ScriptWrappable acts much like a v8::Persistent<> in that it keeps a
@@ -74,7 +98,7 @@ namespace blink {
  *  - disposeWrapper (via setWeakCallback, triggered by V8 garbage collecter):
  *        remove v8::Persistent and install a TypeInfo of the previous value.
  */
-class ScriptWrappable {
+class ScriptWrappable : public ScriptWrappableBase {
 public:
     ScriptWrappable() : m_wrapperOrTypeInfo(0) { }