Pass scroll bar placement as an enum instead of std::string

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 199 matching lines @@
         [painter setKnobStyle:NSScrollerKnobStyleDark];
         break;
     case ScrollbarOverlayStyleLight:
         [painter setKnobStyle:NSScrollerKnobStyleLight];
         break;
     }
 }
 
 ScrollbarButtonsPlacement ScrollbarThemeMacOverlayAPI::buttonsPlacement() const
 {
-    return ScrollbarButtonsNone;
+    return ScrollbarButtonsPlacementNone;
 }
 
 bool ScrollbarThemeMacOverlayAPI::hasThumb(ScrollbarThemeClient* scrollbar)
 {
     ScrollbarPainter painter = painterForScrollbar(scrollbar);
     int minLengthForThumb = [painter knobMinLength] + [painter trackOverlapEndInset] + [painter knobOverlapEndInset]
         + 2 * ([painter trackEndInset] + [painter knobEndInset]);
     return scrollbar->enabled() && (scrollbar->orientation() == HorizontalScrollbar ?
              scrollbar->width() :
              scrollbar->height()) >= minLengthForThumb;
 }
 
 IntRect ScrollbarThemeMacOverlayAPI::backButtonRect(ScrollbarThemeClient* scrollbar, ScrollbarPart part, bool painting)
 {
-    ASSERT(buttonsPlacement() == ScrollbarButtonsNone);
+    ASSERT(buttonsPlacement() == ScrollbarButtonsPlacementNone);
     return IntRect();
 }
 
 IntRect ScrollbarThemeMacOverlayAPI::forwardButtonRect(ScrollbarThemeClient* scrollbar, ScrollbarPart part, bool painting)
 {
-    ASSERT(buttonsPlacement() == ScrollbarButtonsNone);
+    ASSERT(buttonsPlacement() == ScrollbarButtonsPlacementNone);
     return IntRect();
 }
 
 IntRect ScrollbarThemeMacOverlayAPI::trackRect(ScrollbarThemeClient* scrollbar, bool painting)
 {
     ASSERT(!hasButtons(scrollbar));
     return scrollbar->frameRect();
 }
 
 int ScrollbarThemeMacOverlayAPI::minimumThumbLength(ScrollbarThemeClient* scrollbar)
 {
     return [painterForScrollbar(scrollbar) knobMinLength];
 }
 
 void ScrollbarThemeMacOverlayAPI::updateEnabledState(ScrollbarThemeClient* scrollbar)
 {
     [painterForScrollbar(scrollbar) setEnabled:scrollbar->enabled()];
 }
 
 } // namespace blink