Bug 908724 - Avoid #including nsPresContext.h in nsIScrollableFrame.h; r=mats

nsPresContext.h pulls in a large number of headers which should not
be needed in nsIScrollableFrame, if we just move ScrollbarStyles to
its own header

layout/forms/nsListControlFrame.cpp

@@ -594,15 +594,14 @@ nsListControlFrame::ReflowAsDropdown(nsPresContext*           aPresContext,
   return nsHTMLScrollFrame::Reflow(aPresContext, aDesiredSize, state, aStatus);
 }
 
-nsGfxScrollFrameInner::ScrollbarStyles
+ScrollbarStyles
 nsListControlFrame::GetScrollbarStyles() const
 {
   // We can't express this in the style system yet; when we can, this can go away
   // and GetScrollbarStyles can be devirtualized
   int32_t verticalStyle = IsInDropDownMode() ? NS_STYLE_OVERFLOW_AUTO
     : NS_STYLE_OVERFLOW_SCROLL;
-  return nsGfxScrollFrameInner::ScrollbarStyles(NS_STYLE_OVERFLOW_HIDDEN,
-                                                verticalStyle);
+  return ScrollbarStyles(NS_STYLE_OVERFLOW_HIDDEN, verticalStyle);
 }
 
 bool