CC Animation: Use ElementId to attach CC animation players.

cc/trees/layer_tree_host.h

Index: cc/trees/layer_tree_host.h
diff --git a/cc/trees/layer_tree_host.h b/cc/trees/layer_tree_host.h
index 8a522d6008657323ffcc81ab6c04d90ad7b9bce4..4aa6f7c6a2497efd47a1209ffe9f98ee0eed1169 100644
--- a/cc/trees/layer_tree_host.h
+++ b/cc/trees/layer_tree_host.h
@@ -347,6 +347,8 @@ class CC_EXPORT LayerTreeHost : public MutatorHostClient {
   }
 
   Layer* LayerById(int id) const;
+  Layer* LayerForElementId(ElementId element_id) const;
+  Layer* ScrollLayerForElementId(ElementId element_id) const;
 
   void AddLayerShouldPushProperties(Layer* layer);
   void RemoveLayerShouldPushProperties(Layer* layer);
@@ -355,6 +357,9 @@ class CC_EXPORT LayerTreeHost : public MutatorHostClient {
 
   void RegisterLayer(Layer* layer);
   void UnregisterLayer(Layer* layer);
+  void RegisterLayerForElementId(Layer* layer);
+  void UnregisterLayerForElementId(Layer* layer);
+
   // MutatorHostClient implementation.
   bool IsElementInList(ElementId element_id,
                        ElementListType list_type) const override;
@@ -579,6 +584,16 @@ class CC_EXPORT LayerTreeHost : public MutatorHostClient {
 
   using LayerIdMap = std::unordered_map<int, Layer*>;
   LayerIdMap layer_id_map_;
+
+  struct ElementLayers {
+    // Transform, filter and opacity mutations apply to this layer.
+    Layer* main = nullptr;
+    // Scroll mutations apply to this layer.
+    Layer* scroll = nullptr;
+  };
+  using ElementIdToLayerMap = std::unordered_map<ElementId, ElementLayers>;
+  ElementIdToLayerMap element_id_to_layer_map_;
+
   // Set of layers that need to push properties.
   std::unordered_set<Layer*> layers_that_should_push_properties_;