Bug 154009

In JavaScript there are two types of properties - regular value properties, and accessor properties. One difference between these is how they are reflected by getOwnPropertyDescriptor, and another is what object they operate on in the case of a prototype access. If you access a value property of a prototype object it return a value pertinent to the prototype, but in the case of a prototype object returning an accessor, then the accessor function is applied to the base object of the access. JSC supports special 'custom' properties implemented as a c++ callback, and these custom properties can be used to implement either value- or accessor-like behavior. getOwnPropertyDescriptor behavior is selected via the CustomAccessor attribute. Value- or accessor-like object selection is current supported by passing both the slotBase and the thisValue to the callback,and hoping it uses the right one. This is probably inefficient, bug-prone, and leads to crazy like JSBoundSlotBaseFunction. Instead, just pass one thisValue to the callback functions, consistent with CustomAccessor.