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

public/platform/mac/MacScrollTypes.h

Index: public/platform/mac/MacScrollTypes.h
diff --git a/public/platform/mac/MacScrollTypes.h b/public/platform/mac/MacScrollTypes.h
new file mode 100644
index 0000000000000000000000000000000000000000..35a7686d0d062163f8794f7cbb8fb4f56aa51feb
--- /dev/null
+++ b/public/platform/mac/MacScrollTypes.h
@@ -0,0 +1,21 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef MacScrollTypes_h
+#define MacScrollTypes_h
+
+namespace blink {
+
+enum ScrollbarButtonsPlacement {
+    ScrollbarButtonsPlacementNone,
+    ScrollbarButtonsPlacementSingle,
+    ScrollbarButtonsPlacementDoubleStart,
+    ScrollbarButtonsPlacementDoubleEnd,
+    ScrollbarButtonsPlacementDoubleBoth,
+    ScrollbarButtonsPlacementLast = ScrollbarButtonsPlacementDoubleBoth
+};
+
+}
+
+#endif // MacScrollTypes_h