Refactor scrollbar maps from multimap & std::set to simpler flat_{map, set}

cc/layers/scrollbar_layer_impl_base.h

Index: cc/layers/scrollbar_layer_impl_base.h
diff --git a/cc/layers/scrollbar_layer_impl_base.h b/cc/layers/scrollbar_layer_impl_base.h
index d5bc0f8be412b6af942f84ae011b29c4f0917818..7becd72c69e4d89ff8984e80c3357b2b2cd72fca 100644
--- a/cc/layers/scrollbar_layer_impl_base.h
+++ b/cc/layers/scrollbar_layer_impl_base.h
@@ -5,6 +5,7 @@
 #ifndef CC_LAYERS_SCROLLBAR_LAYER_IMPL_BASE_H_
 #define CC_LAYERS_SCROLLBAR_LAYER_IMPL_BASE_H_
 
+#include "base/containers/flat_set.h"
 #include "base/macros.h"
 #include "cc/cc_export.h"
 #include "cc/input/scrollbar.h"
@@ -96,7 +97,7 @@ class CC_EXPORT ScrollbarLayerImplBase : public LayerImpl {
   DISALLOW_COPY_AND_ASSIGN(ScrollbarLayerImplBase);
 };
 
-typedef std::set<ScrollbarLayerImplBase*> ScrollbarSet;
+using ScrollbarSet = base::flat_set<ScrollbarLayerImplBase*>;
 
 }  // namespace cc