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

public/web/mac/WebScrollbarTheme.h

 /*
  * 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 14 matching lines @@
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #ifndef WebScrollbarTheme_h
 #define WebScrollbarTheme_h
 
 #include "public/platform/WebCommon.h"
-
-#include <string>
+#include "public/platform/mac/MacScrollTypes.h"
 
 namespace blink {
 
 // This enum must match NSScrollerStyle in the 10.7 SDK.
 enum ScrollerStyle {
     ScrollerStyleLegacy  = 0,
     ScrollerStyleOverlay = 1
 };
 
 class WebScrollbarTheme {
 public:
     // Updates the scrollbar appearance and behavior.
     // |initialButtonDelay| is the current value of NSScrollerButtonDelay from NSUserDefaults.
     // |autoscrollButtonDelay| is the current value of NSScrollerButtonPeriod from NSUserDefaults.
     // |preferredScrollerStyle| is the current value of +[NSScroller preferredScrollerStyle].
     // |redraw| is true if the update requires a redraw to include the change.
     // |scrollAnimationEnabled| is the current value of NSScrollAnimationEnabled or AppleScrollAnimationEnabled from NSUserDefaults.
     // |buttonPlacement| is the current value of AppleScrollBarVariant
     BLINK_EXPORT static void updateScrollbars(
         float initialButtonDelay, float autoscrollButtonDelay,
         ScrollerStyle preferredScrollerStyle, bool redraw);
     BLINK_EXPORT static void updateScrollbarsWithNSDefaults(
         float initialButtonDelay, float autoscrollButtonDelay,
         ScrollerStyle preferredScrollerStyle, bool redraw,
-        bool scrollAnimationEnabled, const std::string& buttonPlacement);
+        bool scrollAnimationEnabled, ScrollbarButtonsPlacement);
 };
 
 } // namespace blink
 
 #endif