Remove JSObject::propertyIsEnumerable

Reviewed by Sam Weinig.

This method is just a wart - it contains unnecessary const-casting, function call overhead, and LOC.

* runtime/JSObject.cpp:
* runtime/JSObject.h:
    - remove propertyIsEnumerable
* runtime/ObjectPrototype.cpp:
(JSC::objectProtoFuncPropertyIsEnumerable):
    - Move implementation here using getOwnPropertyDescriptor directly.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@154405 268f45cc-cd09-0410-ab3c-d52691b4dbfc