Convert cc::PropertyTree to use a flat_map.

cc/trees/property_tree.h

 // Copyright 2014 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_PROPERTY_TREE_H_
 #define CC_TREES_PROPERTY_TREE_H_
 
 #include <stddef.h>
 
 #include <memory>
 #include <unordered_map>
 #include <vector>
 
+#include "base/containers/flat_map.h"
 #include "cc/base/filter_operations.h"
 #include "cc/base/synced_property.h"
 #include "cc/cc_export.h"
 #include "cc/layers/layer_sticky_position_constraint.h"
 #include "cc/trees/element_id.h"
 #include "ui/gfx/geometry/rect_f.h"
 #include "ui/gfx/geometry/scroll_offset.h"
 #include "ui/gfx/transform.h"
 
 namespace base {
@@ skipping 82 matching lines @@
     int index = FindNodeIndexFromOwningLayerId(id);
     if (index == kInvalidNodeId) {
       DCHECK(property_trees()->is_main_thread);
       property_trees()->needs_rebuild = true;
     }
 
     return Node(index);
   }
 
   int FindNodeIndexFromOwningLayerId(int id) const {
-    auto iter = owning_layer_id_to_node_index.find(id);
-    if (iter == owning_layer_id_to_node_index.end())
+    auto iter = owning_layer_id_to_node_index_.find(id);
+    if (iter == owning_layer_id_to_node_index_.end())
       return kInvalidNodeId;
     else
       return iter->second;
   }
 
   void SetOwningLayerIdForNode(const T* node, int id) {
     if (!node) {
-      owning_layer_id_to_node_index[id] = kInvalidNodeId;
+      owning_layer_id_to_node_index_[id] = kInvalidNodeId;
       return;
     }
 
     DCHECK(node == Node(node->id));
-    owning_layer_id_to_node_index[id] = node->id;
+    owning_layer_id_to_node_index_[id] = node->id;
   }
 
  private:
   std::vector<T> nodes_;
 
-  // These maps map from layer id to the property tree node index.
-  std::unordered_map<int, int> owning_layer_id_to_node_index;
+  // Maps from layer id to the property tree node index. This container is
+  // typically very small and the memory overhead of unordered_map will
+  // dominate so use a flat_map. See http://crbug.com/709243
+  base::flat_map<int, int> owning_layer_id_to_node_index_;
 
   bool needs_update_;
   PropertyTrees* property_trees_;
 };
 
 struct StickyPositionNodeData;
 
 class CC_EXPORT TransformTree final : public PropertyTree<TransformNode> {
  public:
   TransformTree();
@@ skipping 572 matching lines @@
   DrawTransforms& GetDrawTransforms(int transform_id, int effect_id) const;
   DrawTransformData& FetchDrawTransformsDataFromCache(int transform_id,
                                                       int effect_id) const;
 
   PropertyTreesCachedData cached_data_;
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_PROPERTY_TREE_H_