[webkit-dev] ScrollbarPart and the way it is used

[Darin Adler]

I believe this is Hyatt's design:

ScrollbarPart is used when the argument is going to be a single part.  
ScrollbarControlPartMask is used when the argument is a set of parts.

The same enum values, from the definition of ScrollbarPart, are used  
for both, but you can tell from the type used what the programmer's  
intent is.

     -- Darin