Replace HandleWrapper and avoid repoint function

JS::Handle<T>::repoint gets removed with SpiderMonkey 38, so the
existing solution has to be replaced. The new approach should also be a
bit clearer. Named Return Value Optimization (NRVO) should avoid a
superfluous temporary for the return value in the generic template
function implementation of AssignOrFromJSVal.

Refs #3708

This was SVN commit r17695.

source/scriptinterface/ScriptInterface.h

@@ -375,6 +375,14 @@ public:
 	{
 		AssignOrToJSVal(cx, handle, a);
 	}
+	
+	/**
+	 * Converts |val| to T if needed or just returns it if it's a handle.
+	 * This is meant for use in other templates where we want to use the same code for JS::HandleValue and
+	 * other types.
+	 */
+	template <typename T>
+	static T AssignOrFromJSVal(JSContext* cx, const JS::HandleValue& val, bool& ret);
 
 private:
 	
@@ -476,6 +484,21 @@ inline void ScriptInterface::AssignOrToJSValUnrooted<JS::Value>(JSContext* UNUSE
 	handle.set(a);
 }
 
+template<typename T>
+inline T ScriptInterface::AssignOrFromJSVal(JSContext* cx, const JS::HandleValue& val, bool& ret)
+{
+	T retVal;
+	ret = FromJSVal(cx, val, retVal);
+	return retVal;
+}
+
+template<>
+inline JS::HandleValue ScriptInterface::AssignOrFromJSVal<JS::HandleValue>(JSContext* UNUSED(cx), const JS::HandleValue& val, bool& ret)
+{
+	ret = true;
+	return val;
+}
+
 template<typename T0>
 bool ScriptInterface::CallFunctionVoid(JS::HandleValue val, const char* name, const T0& a0)
 {