CC Animations: Establish AnimationHost, AnimationTimeline and AnimationPlayer

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index 4093c425eb19541fc3a05651e0318a13c1313eed..446758b56c3dc39c5d03dbcf1aed8ab2b03fc08d 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -697,8 +697,8 @@ gfx::Size LayerTreeImpl::ScrollableSize() const {
   return root_scroll_layer->children()[0]->bounds();
 }
 
-LayerImpl* LayerTreeImpl::LayerById(int id) {
-  LayerIdMap::iterator iter = layer_id_map_.find(id);
+LayerImpl* LayerTreeImpl::LayerById(int id) const {
+  LayerIdMap::const_iterator iter = layer_id_map_.find(id);
   return iter != layer_id_map_.end() ? iter->second : NULL;
 }