Move preferred scrollbar style preference change listening from renderer to browser, 3 of 3.

Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 67 matching lines @@
     updateEnabledState(scrollbar);
     updateScrollbarOverlayStyle(scrollbar);
 }
 
 ScrollbarPainter ScrollbarThemeMacOverlayAPI::painterForScrollbar(ScrollbarThemeClient* scrollbar)
 {
     return scrollbarPainterMap()->get(scrollbar).get();
 }
 
 void ScrollbarThemeMacOverlayAPI::paintTrackBackground(GraphicsContext* context, ScrollbarThemeClient* scrollbar, const IntRect& rect) {
-    ASSERT(isScrollbarOverlayAPIAvailable());
+    ASSERT(isOverlayAPIAvailable());
 
     GraphicsContextStateSaver stateSaver(*context);
     context->translate(rect.x(), rect.y());
     LocalCurrentGraphicsContext localContext(context);
 
     CGRect frameRect = scrollbar->frameRect();
     ScrollbarPainter scrollbarPainter = painterForScrollbar(scrollbar);
     [scrollbarPainter setEnabled:scrollbar->enabled()];
     [scrollbarPainter setBoundsSize: NSSizeFromCGSize(frameRect.size)];
 
     NSRect trackRect = NSMakeRect(0, 0, frameRect.size.width, frameRect.size.height);
     [scrollbarPainter drawKnobSlotInRect:trackRect highlight:NO];
 }
 
 void ScrollbarThemeMacOverlayAPI::paintThumb(GraphicsContext* context, ScrollbarThemeClient* scrollbar, const IntRect& rect) {
-    ASSERT(isScrollbarOverlayAPIAvailable());
+    ASSERT(isOverlayAPIAvailable());
 
     GraphicsContextStateSaver stateSaver(*context);
     context->translate(rect.x(), rect.y());
     LocalCurrentGraphicsContext localContext(context);
 
     ScrollbarPainter scrollbarPainter = painterForScrollbar(scrollbar);
     [scrollbarPainter setEnabled:scrollbar->enabled()];
     [scrollbarPainter setBoundsSize:NSSizeFromCGSize(rect.size())];
     [scrollbarPainter setDoubleValue:0];
     [scrollbarPainter setKnobProportion:1];
@@ skipping 71 matching lines @@
 {
     return [painterForScrollbar(scrollbar) knobMinLength];
 }
 
 void ScrollbarThemeMacOverlayAPI::updateEnabledState(ScrollbarThemeClient* scrollbar)
 {
     [painterForScrollbar(scrollbar) setEnabled:scrollbar->enabled()];
 }
 
 } // namespace WebCore