cc : Replace unordered_maps with flat_map in property trees

Because these maps should be small.

BUG=678397
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2848243003
Cr-Commit-Position: refs/heads/master@{#468403}

cc/trees/property_tree.cc

@@ -1361,11 +1361,9 @@ SyncedScrollOffset* ScrollTree::GetOrCreateSyncedScrollOffset(int layer_id) {
 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 {

cc/trees/property_tree.h

@@ -507,9 +507,9 @@ class CC_EXPORT ScrollTree final : public PropertyTree<ScrollNode> {
   ScrollNode* FindNodeFromElementId(ElementId id);
 
  private:
-  using ScrollOffsetMap = std::unordered_map<int, gfx::ScrollOffset>;
+  using ScrollOffsetMap = base::flat_map<int, gfx::ScrollOffset>;
   using SyncedScrollOffsetMap =
-      std::unordered_map<int, scoped_refptr<SyncedScrollOffset>>;
+      base::flat_map<int, scoped_refptr<SyncedScrollOffset>>;
 
   int currently_scrolling_node_id_;
 
@@ -646,12 +646,9 @@ class CC_EXPORT PropertyTrees final {
   // from layer id to the respective property node. Completing that work is
   // pending the launch of Slimming Paint v2 and reworking UI compositor logic
   // to produce cc property trees and these maps.
-  std::unordered_map<ElementId, int, ElementIdHash>
-      element_id_to_effect_node_index;
-  std::unordered_map<ElementId, int, ElementIdHash>
-      element_id_to_scroll_node_index;
-  std::unordered_map<ElementId, int, ElementIdHash>
-      element_id_to_transform_node_index;
+  base::flat_map<ElementId, int> element_id_to_effect_node_index;
+  base::flat_map<ElementId, int> element_id_to_scroll_node_index;
+  base::flat_map<ElementId, int> element_id_to_transform_node_index;
 
   std::vector<int> always_use_active_tree_opacity_effect_ids;
   TransformTree transform_tree;