[gin] Introduce Wrappable::GetObjectTemplate

gin/function_template.h.pump

Index: gin/function_template.h.pump
diff --git a/gin/function_template.h.pump b/gin/function_template.h.pump
index fad718febc277ef1e2566fe0eb49ad5ed70c4776..e65aeb889bf41505afa883454aaa05e4b47fe15d 100644
--- a/gin/function_template.h.pump
+++ b/gin/function_template.h.pump
@@ -68,6 +68,10 @@ class GIN_EXPORT CallbackHolderBase : public Wrappable<CallbackHolderBase> {
 
  protected:
   virtual ~CallbackHolderBase() {}
+
+ private:
+  friend class Wrappable<CallbackHolderBase>;
+  static v8::Local<v8::ObjectTemplate> GetObjectTemplate(v8::Isolate* isolate);
 };
 
 template<typename Sig>
@@ -187,11 +191,6 @@ $for ARG [[    typename CallbackParamTraits<P$(ARG)>::LocalType a$(ARG);
 }  // namespace internal
 
 
-// This should be called once per-isolate to initialize the function template
-// system.
-GIN_EXPORT void InitFunctionTemplates(PerIsolateData* isolate_data);
-
-
 // CreateFunctionTemplate creates a v8::FunctionTemplate that will create
 // JavaScript functions that execute a provided C++ function or base::Callback.
 // JavaScript arguments are automatically converted via gin::Converter, as is