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

cc/trees/layer_tree_impl.h

 // Copyright 2012 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_TREES_LAYER_TREE_IMPL_H_
 #define CC_TREES_LAYER_TREE_IMPL_H_
 
 #include <map>
 #include <set>
 #include <string>
@@ skipping 560 matching lines @@
   std::unordered_map<ElementId, gfx::Transform, ElementIdHash>
       element_id_to_transform_animations_;
   std::unordered_map<ElementId, FilterOperations, ElementIdHash>
       element_id_to_filter_animations_;
 
   // Maps from clip layer ids to scroll layer ids.  Note that this only includes
   // the subset of clip layers that act as scrolling containers.  (This is
   // derived from LayerImpl::scroll_clip_layer_ and exists to avoid O(n) walks.)
   std::unordered_map<int, int> clip_scroll_map_;
 
-  // Maps scroll element ids to scrollbar layer ids. For each scroll layer,
-  // there may be 1 or 2 scrollbar layers (for vertical and horizontal). (This
-  // is derived from ScrollbarLayerImplBase::scroll_element_id_ and exists to
-  // avoid O(n) walks.)
-  // TODO(pdr): Refactor this to be more efficient--likely a map where the value
-  // is a pair of scrollbar layer ids instead of using a multimap.
-  std::multimap<ElementId, int> element_id_to_scrollbar_layer_ids_;
+  struct ScrollbarLayerIds {
+    int horizontal = Layer::INVALID_ID;
+    int vertical = Layer::INVALID_ID;
+  };
+  // Each scroll layer can have up to two scrollbar layers (vertical and
+  // horizontal). This mapping is maintained as part of scrollbar registration.
+  base::flat_map<ElementId, ScrollbarLayerIds>
+      element_id_to_scrollbar_layer_ids_;
 
   std::vector<PictureLayerImpl*> picture_layers_;
   LayerImplList surface_layers_;
 
   // List of render surfaces for the most recently prepared frame.
   RenderSurfaceList render_surface_list_;
   // After drawing the |render_surface_list_| the areas in this region
   // would not be fully covered by opaque content.
   Region unoccluded_screen_space_region_;
 
@@ skipping 35 matching lines @@
   // lifecycle states. See: |LayerTreeLifecycle|.
   LayerTreeLifecycle lifecycle_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(LayerTreeImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_IMPL_H_