cc: Add id->LayerImpl map to LayerTreeImpl

cc/layer_tree_impl.cc

Index: cc/layer_tree_impl.cc
diff --git a/cc/layer_tree_impl.cc b/cc/layer_tree_impl.cc
index d434bb0e6a2a4f78f1706fa3740720224f8bc1d0..a113316c77b3dc79bac020ad0c65d8b3af0df422 100644
--- a/cc/layer_tree_impl.cc
+++ b/cc/layer_tree_impl.cc
@@ -19,6 +19,9 @@ LayerTreeImpl::LayerTreeImpl(LayerTreeHostImpl* layer_tree_host_impl)
 }
 
 LayerTreeImpl::~LayerTreeImpl() {
+  // Need to explicitly clear the tree prior to destroying this so that
+  // the LayerTreeImpl pointer is still valid in the LayerImpl dtor.
+  root_layer_.reset();
 }
 
 static LayerImpl* findRootScrollLayer(LayerImpl* layer)
@@ -68,6 +71,21 @@ void LayerTreeImpl::ClearCurrentlyScrollingLayer() {
   scrolling_layer_id_from_previous_tree_ = 0;
 }
 
+LayerImpl* LayerTreeImpl::LayerById(int id) {
+  LayerIdMap::iterator iter = layer_id_map_.find(id);
+  return iter != layer_id_map_.end() ? iter->second : NULL;
+}
+
+void LayerTreeImpl::RegisterLayer(LayerImpl* layer) {
+  DCHECK(!LayerById(layer->id()));
+  layer_id_map_[layer->id()] = layer;
+}
+
+void LayerTreeImpl::UnregisterLayer(LayerImpl* layer) {
+  DCHECK(LayerById(layer->id()));
+  layer_id_map_.erase(layer->id());
+}
+
 const LayerTreeSettings& LayerTreeImpl::settings() const {
   return layer_tree_host_impl_->settings();
 }