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

cc/trees/layer_tree_impl.h

Index: cc/trees/layer_tree_impl.h
diff --git a/cc/trees/layer_tree_impl.h b/cc/trees/layer_tree_impl.h
index 46f57fda5b625c775e9c8d0bb1647948137ce6c7..ccff3c2447bfcb393ab6ae4e38a813d4c3dec2b4 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -578,13 +578,14 @@ class CC_EXPORT LayerTreeImpl {
   // 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_;