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

Source/platform/scroll/ScrollbarThemeMacCommon.mm

 /*
  * Copyright (C) 2008, 2011 Apple 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 145 matching lines @@
     IntRect tickmarkTrackRect = rect;
     tickmarkTrackRect.setX(tickmarkTrackRect.x() + 1);
     tickmarkTrackRect.setWidth(tickmarkTrackRect.width() - 2);
     paintGivenTickmarks(context->canvas(), scrollbar, tickmarkTrackRect, tickmarks);
 }
 
 ScrollbarThemeMacCommon::~ScrollbarThemeMacCommon()
 {
 }
 
-void ScrollbarThemeMacCommon::preferencesChanged(float initialButtonDelay, float autoscrollButtonDelay, NSScrollerStyle preferredScrollerStyle, bool redraw, bool scrollAnimationEnabled, const std::string& buttonPlacement)
+void ScrollbarThemeMacCommon::preferencesChanged(float initialButtonDelay, float autoscrollButtonDelay, NSScrollerStyle preferredScrollerStyle, bool redraw, bool scrollAnimationEnabled, ScrollbarButtonsPlacement buttonPlacement)
 {
     updateButtonPlacement(buttonPlacement);
     gInitialButtonDelay = initialButtonDelay;
     gAutoscrollButtonDelay = autoscrollButtonDelay;
     gScrollAnimationEnabledForSystem = scrollAnimationEnabled;
     bool sendScrollerStyleNotification = gPreferredScrollerStyle != preferredScrollerStyle;
     gPreferredScrollerStyle = preferredScrollerStyle;
     if (redraw && !scrollbarSet().isEmpty()) {
         ScrollbarSet::iterator end = scrollbarSet().end();
         for (ScrollbarSet::iterator it = scrollbarSet().begin(); it != end; ++it) {
@@ skipping 58 matching lines @@
 // static
 bool ScrollbarThemeMacCommon::isOverlayAPIAvailable()
 {
     static bool apiAvailable =
         [NSClassFromString(@"NSScrollerImp") respondsToSelector:@selector(scrollerImpWithStyle:controlSize:horizontal:replacingScrollerImp:)]
         && [NSClassFromString(@"NSScrollerImpPair") instancesRespondToSelector:@selector(scrollerStyle)];
     return apiAvailable;
 }
 
 } // namespace blink