Add the support for Symbol attributes on IDL

Add the support for Symbol attributes on IDL
https://bugs.webkit.org/show_bug.cgi?id=150586

Reviewed by Ryosuke Niwa.

This patch addes readonly attribute support for Symbols.
It involves the IDL generator functionality converting Native type (PrivateName) to Symbol.

* bindings/scripts/CodeGeneratorGObject.pm:
(SkipAttribute):
(SkipFunction):
* bindings/scripts/CodeGeneratorJS.pm:
(NativeToJSValue):
* bindings/scripts/CodeGeneratorObjC.pm:
(SkipFunction):
(SkipAttribute):
* bindings/scripts/test/JS/JSTestObj.cpp:
(WebCore::jsTestObjReadOnlySymbolAttr):
(WebCore::jsTestObjConstructorStaticReadOnlySymbolAttr):
* bindings/scripts/test/TestObj.idl:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@191841 268f45cc-cd09-0410-ab3c-d52691b4dbfc