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

cc/layers/scrollbar_layer_impl_base.h

 // Copyright 2013 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 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"
 #include "cc/layers/layer.h"
 #include "cc/layers/layer_impl.h"
 
 namespace cc {
 
 class LayerTreeImpl;
 
@@ skipping 71 matching lines @@
   ScrollbarOrientation orientation_;
   bool is_left_side_vertical_scrollbar_;
 
   // Difference between the clip layer's height and the visible viewport
   // height (which may differ in the presence of top-controls hiding).
   float vertical_adjust_;
 
   DISALLOW_COPY_AND_ASSIGN(ScrollbarLayerImplBase);
 };
 
-typedef std::set<ScrollbarLayerImplBase*> ScrollbarSet;
+using ScrollbarSet = base::flat_set<ScrollbarLayerImplBase*>;
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_SCROLLBAR_LAYER_IMPL_BASE_H_