cc : Replace unordered_maps with flat_map in property trees

cc/trees/property_tree.cc

 // 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.
 
 #include <stddef.h>
 
 #include <set>
 #include <vector>
 
 #include "base/logging.h"
@@ skipping 1343 matching lines @@
   if (layer_id_to_synced_scroll_offset_map_.find(layer_id) ==
       layer_id_to_synced_scroll_offset_map_.end()) {
     layer_id_to_synced_scroll_offset_map_[layer_id] = new SyncedScrollOffset;
   }
   return layer_id_to_synced_scroll_offset_map_[layer_id].get();
 }
 
 const SyncedScrollOffset* ScrollTree::GetSyncedScrollOffset(
     int layer_id) const {
   DCHECK(!property_trees()->is_main_thread);
-  if (layer_id_to_synced_scroll_offset_map_.find(layer_id) ==
-      layer_id_to_synced_scroll_offset_map_.end()) {
-    return nullptr;
-  }
-  return layer_id_to_synced_scroll_offset_map_.at(layer_id).get();
+  auto it = layer_id_to_synced_scroll_offset_map_.find(layer_id);
+  return it != layer_id_to_synced_scroll_offset_map_.end() ? it->second.get()
+                                                           : nullptr;
 }
 
 const gfx::ScrollOffset ScrollTree::current_scroll_offset(int layer_id) const {
   if (property_trees()->is_main_thread) {
     ScrollOffsetMap::const_iterator it =
         layer_id_to_scroll_offset_map_.find(layer_id);
     return it != layer_id_to_scroll_offset_map_.end() ? it->second
                                                       : gfx::ScrollOffset();
   }
   return GetSyncedScrollOffset(layer_id)
@@ skipping 765 matching lines @@
   const EffectNode* effect_node = effect_tree.Node(effect_id);
 
   if (effect_node->surface_contents_scale.x() != 0.0 &&
       effect_node->surface_contents_scale.y() != 0.0)
     screen_space_transform.Scale(1.0 / effect_node->surface_contents_scale.x(),
                                  1.0 / effect_node->surface_contents_scale.y());
   return screen_space_transform;
 }
 
 }  // namespace cc