Revert of cc: Add a main thread LayerListIterator

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 7d178db1e66ee257847d74b705521255c6686e75..5fbc305b3c13d5cc2bd397b400f5e5b40a5a148d 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -441,20 +441,20 @@
   target_tree->has_ever_been_drawn_ = false;
 }
 
-LayerListIterator<LayerImpl> LayerTreeImpl::begin() {
-  return LayerListIterator<LayerImpl>(root_layer_);
-}
-
-LayerListIterator<LayerImpl> LayerTreeImpl::end() {
-  return LayerListIterator<LayerImpl>(nullptr);
-}
-
-LayerListReverseIterator<LayerImpl> LayerTreeImpl::rbegin() {
-  return LayerListReverseIterator<LayerImpl>(root_layer_);
-}
-
-LayerListReverseIterator<LayerImpl> LayerTreeImpl::rend() {
-  return LayerListReverseIterator<LayerImpl>(nullptr);
+LayerListIterator LayerTreeImpl::begin() {
+  return LayerListIterator(root_layer_);
+}
+
+LayerListIterator LayerTreeImpl::end() {
+  return LayerListIterator(nullptr);
+}
+
+LayerListReverseIterator LayerTreeImpl::rbegin() {
+  return LayerListReverseIterator(root_layer_);
+}
+
+LayerListReverseIterator LayerTreeImpl::rend() {
+  return LayerListReverseIterator(nullptr);
 }
 
 void LayerTreeImpl::AddToElementMap(LayerImpl* layer) {