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

Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm

Index: Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm
diff --git a/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm b/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm
index 321a8954796cb1384c3b1a959dde86b12f57cb98..2046a60997ebd8e5d2a3000321b5bc3f670887bc 100644
--- a/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm
+++ b/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm
@@ -217,7 +217,7 @@ void ScrollbarThemeMacOverlayAPI::updateScrollbarOverlayStyle(ScrollbarThemeClie
 
 ScrollbarButtonsPlacement ScrollbarThemeMacOverlayAPI::buttonsPlacement() const
 {
-    return ScrollbarButtonsNone;
+    return ScrollbarButtonsPlacementNone;
 }
 
 bool ScrollbarThemeMacOverlayAPI::hasThumb(ScrollbarThemeClient* scrollbar)
@@ -232,13 +232,13 @@ bool ScrollbarThemeMacOverlayAPI::hasThumb(ScrollbarThemeClient* scrollbar)
 
 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();
 }