cc: Use property tree index accessors in Layer::On<Property>Animated

cc/layers/layer.cc

Index: cc/layers/layer.cc
diff --git a/cc/layers/layer.cc b/cc/layers/layer.cc
index 48d932d4c2f53fed0d363293f3b7b864e5f82ffa..60145c9eb5dda88011259d3a55759ecce6528fa2 100644
--- a/cc/layers/layer.cc
+++ b/cc/layers/layer.cc
@@ -1408,7 +1408,7 @@ void Layer::OnOpacityAnimated(float opacity) {
   if (layer_tree_host_) {
     if (OpacityNode* node =
             layer_tree_host_->property_trees()->opacity_tree.Node(
-                opacity_tree_index_)) {
+                opacity_tree_index())) {
       if (node->owner_id == id())
         node->data = opacity;
     }
@@ -1423,7 +1423,7 @@ void Layer::OnTransformAnimated(const gfx::Transform& transform) {
   if (layer_tree_host_) {
     if (TransformNode* node =
             layer_tree_host_->property_trees()->transform_tree.Node(
-                transform_tree_index_)) {
+                transform_tree_index())) {
       if (node->owner_id == id()) {
         node->data.local = transform;
         node->data.needs_local_transform_update = true;