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

Source/platform/scroll/ScrollbarThemeMacCommon.h

 /*
  * Copyright (C) 2008 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 23 matching lines @@
 namespace blink {
 
 class Pattern;
 
 class PLATFORM_EXPORT ScrollbarThemeMacCommon : public ScrollbarTheme {
 public:
     ~ScrollbarThemeMacCommon() override;
 
     void registerScrollbar(ScrollbarThemeClient*) override;
     void unregisterScrollbar(ScrollbarThemeClient*) override;
-    void preferencesChanged(float initialButtonDelay, float autoscrollButtonDelay, NSScrollerStyle preferredScrollerStyle, bool redraw, bool scrollAnimationEnabled, const std::string& buttonPlacement);
+    void preferencesChanged(float initialButtonDelay, float autoscrollButtonDelay, NSScrollerStyle preferredScrollerStyle, bool redraw, bool scrollAnimationEnabled, ScrollbarButtonsPlacement);
 
     bool supportsControlTints() const override { return true; }
 
     double initialAutoscrollTimerDelay() override;
     double autoscrollTimerDelay() override;
 
     void paintTickmarks(GraphicsContext*, ScrollbarThemeClient*, const IntRect&) override;
 
     static NSScrollerStyle recommendedScrollerStyle();
 
     static bool isOverlayAPIAvailable();
 
     static bool scrollAnimationEnabledForSystem();
 
 protected:
     int maxOverlapBetweenPages() override { return 40; }
 
     bool shouldDragDocumentInsteadOfThumb(ScrollbarThemeClient*, const PlatformMouseEvent&) override;
     int scrollbarPartToHIPressedState(ScrollbarPart);
 
-    virtual void updateButtonPlacement(const std::string& buttonPlacement) {}
+    virtual void updateButtonPlacement(ScrollbarButtonsPlacement) {}
 
     void paintGivenTickmarks(SkCanvas*, ScrollbarThemeClient*, const IntRect&, const Vector<IntRect>&);
 
     RefPtr<Pattern> m_overhangPattern;
 };
 
 }
 
 #endif // ScrollbarThemeMacCommon_h